(The Inflicter of Death)
The Slayer, The Bringer of Death, The Life-Taker.
Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is Al-Mumeet (in Arabic: ٱلْمُمِيتُ), meaning the one who gives life and the one who takes it away. He ordains who will become lifeless. Eventually, we all return to Him.
From the root mim-waw-ta (م و ت), which has the following classical Arabic connotations: to die, pass away, burn out, be lifeless, to be quiet, still, calm, to be inanimate, to be deprived of sensation, to be spiritually dead, lacking spiritual life.
Among the disputed names:
As we've mentioned prior, different scholars have different criteria for what qualifies as a name of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ. Of the 99 names, 81 are explicitly mentioned in the Qur'an. Therefore, there is not one agreed-upon list for the remaining 18 names. Al-Mumeet is one of those names which is not included by certain scholars. This list includes Ibn al-Wazir, Ibn Hazm, and Ibn al-Uthaymeen, among others. However, others, such as Ibn Arabi, Imam al-Bayhaqi, and al-Ghazali, have included this name in their lists.
The pairing of Al-Muhyi and Al-Mumeet:
The names Al-Muhyi and Al-Mumeet are often mentioned together for their contrasting nature. Al-Muhyi is the bestower of life, and Al-Mumeet is the bringer of death. These two attributes are not shared with anyone and are reserved for Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ alone.
Al-Muhyi and Al-Mumeet are necessary as they bring balance to all things. If we lived as if we were immortal, we would be more likely overtaken by the temptations of this world with fear of no consequences. But the knowledge of our mortality softens many emotions and balances us out. We should have no reason to fear death; death is a gift to the believer. Because of this constraint, we rush to do good, seek Allah's forgiveness, and offer our daily prayers. The duality of life and death is created by Al-Muhyi and Al-Mumeet, which maintains this essential balance.
Wa mal hayaatud dunyaaa illaa la'ibunw wa lahwunw wa lad Daarul Aakhiratu khaiyrul lillazeena yattaqoon; afalaa ta'qiloon
"And the worldly life is not but amusement and diversion; but the home of the Hereafter is best for those who fear Allah, so will you not reason?" — (Qur'an 6:32)
Death is not non-existence:
These verses from the Qur'an do not include the name Al-Mumeet but help us understand the quality of being called the slayer. "Abdallah b. Amr reported God's Messenger ﷺ as saying: 'The gift to a believer is death.' (i.e., death is the means by which he gains an introduction to the delights of paradise.)"  Death is a state of being for living things. It does not mean non-existence or nothingness. It exists as another state from which our lives transcend.
Some scholars have commented on how we live many lives. But they are all connected, one continuous life compartmentalized into distinct phases. We have the life of this world (hayat al dunya), life of the grave (hayat al qabr), life of the interworld (hayat al barzakh), the life of the day resurrection (hayat al qiyamah), and the final destination the life of the hereafter (hayat al akhirah).
Wa likulli ummatin ajalun fa izaa jaaa'a ajaluhum laa yasta' khiroona saa'atanw wa laa yastaqdimoon
"And for every nation is a [specified] term. So when their time has come, they will not remain behind an hour, nor will they precede [it]." — (Qur'an 7:34)
Abdullah b. Amr b. al-'As reported: I heard Allah's Messenger (ﷺ) as saying: Allah ordained the measures (of quality) of the creation fifty thousand years before He created the heavens and the earth, as His Throne was upon water. 
Lahoo mulkus samaawaati wal ardi yuhyee wa yumeetu wa Huwa 'alaa kulli shai'in Qadeer
"His is the dominion of the heavens and earth. He gives life and causes death, and He is over all things competent." — (Qur'an 57:2)
Narrated Anas bin Malik: The Prophet ﷺ said, "None of you should wish for death because of a calamity befalling him; but if he has to wish for death, he should say: "O Allah! Keep me alive as long as life is better for me, and let me die if death is better for me.'" 
The Prophet ﷺ tells us to avoid making statements that shows displeasure with the decree of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ. We should avoid jokingly or sarcastically saying, "I would like nothing more than to just die." Instead, we are told of the rewards that are offered to the believer who is patient when tragedy strikes. (1) Expiation of His or Her sins.  (2) Those who are patient for the sake of Allah in anticipation of receiving his mercy and blessings will be rewarded. 
Huwa yuhyee wa yumeetu wa ilaihi turja'oon
"He gives life and causes death, and to Him you will be returned" — (Qur'an 10:56)
Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is the one who gives (Al-Muhyi) and takes (Al-Mumeet). To Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ belong all things when He causes them to return - He is retrieving what was rightfully His. In The Name & The Named, it mentions how the human body is made from a combination of flesh and soul. The flesh is visible; the soul is not. The body is temporal (short-lasting), whereas the soul is eternal. The book provides a useful conceptualization: the soul with its body is like a merchant who buys and sells goods, making gains and losses. When the soul parts from the body, it's like the business which no longer serves its owner. When we die, we are left with whatever profit or debt we have incurred. In the grave, we wait for the day of judgment, either bankrupt or rich.
Innal laaha indahoo 'ilmus saa'ati wa yunazzilul ghaisa wa ya'lamu maa fil arhaami wa maa tadree nafsum maazaa taksibu ghadaa; wa maa tadree nafsum bi ayyi ardin tamoot; innal laaha 'Aleemun Khabeer (section 4)
"Indeed, Allah [alone] has knowledge of the Hour and sends down the rain and knows what is in the wombs. And no soul perceives what it will earn tomorrow, and no soul perceives in what land it will die. Indeed, Allah is Knowing and Acquainted." — (Qur'an 31:34)
Usamah bin Zaid (May Allah be pleased with them) reported: A daughter of the Prophet ﷺ sent a message to him that her son was at his last breath and requested him to come to her. The Messenger of Allah ﷺ sent back the informer saying: "To Allah belongs what He takes and what He gives, and everything has a limited period (in this world). So ask her to endure patiently, and expect the reward of Allah." 
Du'a calling on this attribute of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ:
Abu Ayyub Al-Ansari narrated that the Messenger of Allah (ﷺ) said: “Whoever says ten times: (reciting the du'a below) it is for him equal to freeing four slaves among the offspring of Isma'il.”
La ilaha illallah, wahdahu la sharika lahu, lahul-mulku wa lahul-hamdu, yuhyi wa yumitu wa huwa 'ala kulli shai'in qadir
"None has the right to be worshipped but Allah alone, Who has no partner. His is the dominion and His is the praise. He brings life and He causes death, and He is Able to do all things." 
 Mishkat al-Masabih 1609
 Sahih Muslim 2653b
 Sahih al-Bukhari 5671
 Sahih al-Bukhari 5640 | No calamity befalls a Muslim but that Allah expiates some of his sins because of it, even though it were the prick he receives from a thorn.
 Al-Adab Al-Mufrad 532 & Qur'an 25:75 | "Those will be awarded the Chamber for what they patiently endured, and they will be received therein with greetings and [words of] peace."
 Riyad as-Salihin 924
 Sahih (Darussalam) Jami at-Tirmidhi 3553