I strongly agree with TwinRetro - this is not a guessing game, the stakes are far too high. You have a killer of 14 people on your hands (12? I didn't read the updated info yet) that injured 30, at whom he opened fire, he also detonated explosives in a public, crowded place.
Let's say he goes to rehab. Let's imagine for a moment that the rehab fails and he does it again. What do you do? Say "Oupsie! Well, he wasn't "fully cured", let's put him on some more rehab." and try again?
Are you willing to carry those 14 deaths and 30 injuries that may turn into a higher deathtoll in, say, 20 years on your conscience because you wanted to be noble and you value life?