Firstly i recommend you to explore all the evidence,if you`ll do that you will get a higher chance to kill him(But you can try without it)
1.Take the pistol from the dead officer


2.Go out and talk with Deviant,here`s how you need to talk with him
Firsly say Hello Daniel,then choose calm button,then dont apply the tourniquete and choose the obey button
3.Then he will ask you if you have a gun,say that you dont have a gun(Choose lie)
4.And then say about him and Emma that they were very close and other things
5.Pick the «Defective» word
6.Then press the button «Talk to the Hostage »
7.Then say to the soldiers that situatioin is under control and that you dont need them(so they can fly away)
8.Pick «Trust» button
9.Then press Compromise button and then press the button «Use Gun »
10.Execute Him
Here you go!You saved a child without getting murdered
there are also ending in which snipers are killing this deviant(i will add this ending later)