I don't "feel for him" either, but take an objective look on the matter.
Fact number one is that he had to be given treatment both from legal point of view - not doing it would've been involuntary manslaughter by criminal negligence, as well as some other offenses related on abuse of positions and so on... it's all based on Montesquieu's separation of powers - police force belonging to the executive branch, it doesn't have any judicial power and thus is only allowed to very limited amount of "field justice", mainly in the use of power to neutralize hostiles (or should we say, "facilitate communications and terminate hostilities"...

). Against an injured person (suspect or otherwise) who is of no threat, the police doesn't have technically any power. Heck, you can even refuse to pay a speeding ticket and bring it on court AFAIK, in which you will be made to pay the ticket as well as the judiciary expenses, but you
can do it - the ticket given by an officer isn't legally binding except if you accept it to be so, at least according to my understanding... but I'm getting on a tangent again.
Not to mention the fact that there wasn't really any surefire way to know that he indeed was the gunman at that point - strong suspicion, maybe, but no more.
Let's take a hypothetic situation. You stumble upon a grievous sight on some street or where-ever; there's ~10 people bleeding, unconscious and at varying condition depending on where they were injured, or dead. Clearly the injuries are bullet wounds. One person has a discharged pistol close to his or her hand and a bullet wound on head, but is still alive. Naturally, you call the paramedics and police and start the treatment of those alive by the best of your ability and possibly try to shout some people to help, but that is not of consequence.
Can you by some incredible intuition surely know that the one close to the gun is indeed the one responsible for the gory sight upon you? And that he or she is not another victim and the gun was placed close to him or her because the wound happened to look like it could be self-inflicted? You could discern it with forensic evidence, but that takes time and the profusively bleeding victims don't have that.
Even if there was no question, like if some of the injured were conscious enough to relay the message of the shooter's identity, would it really be ethically acceptably to leave the person entirely untreated? From my point of view, no. But then again, I don't believe in death as a method of punishment - it makes no sense since it doesn't actually punish anyone, it's just a legally practiced form of revenge where it's practiced...
Mainly because I cannot become an authority to decide who lives and who dies, it's out of my ability - similarly I can't really have any authority to decide who should live or die. Tolkien really summed it up pretty well in Gandalf's mouth as quoted in my previous message.
As to his treatment taking resources - well, tough. It can't really be helped. Such is the matter with every criminal, they take a lot of maintenance (food, quarters, healthcare etc.), and from strictly financial point of view, death penalty would indeed be a good way to simply get rid of the criminals - but it would take an infallible legal system to have even some [utilitarian] ethical argument behind it. And even then I'd regard it as a rather barbaric form of criminal management.
Would you support offing every criminal who takes resources from others, "more deserving" citizens?
