Depending on the type of explosive used in the bomb belt, hitting it with a bullet may or may not have triggered it. It's all a matter of safety. You don't want something that is going to detonate if you fall down or something hits it. You want it to only detonate when you tell it to. It's like how the original dynamite ignited via a (hopefully long) fuse while in later years they switched to a type that was ignited via an electrical plunger. This type would not detonate in a fire and was much safer.

If I recall correctly, C4 won't detonate when it's hit or shot but gives you a pretty good bang for your buck when detonated properly. C4 is something that you could use in a bomb belt quite effectively.

But your main point was this, the cops were pretty stupid to shoot the guy and risk hitting the belt. I'm guessing they had a reason for shooting him. Maybe he was trying to detonate the belt. Better to kill him then have him detonate the belt and take out a lot of people and/or infrastructure.

I have no sympathy for terrorists. I don't know all of the details on this particular incident but if the person was wearing a bomb belt and the police were 100% sure that's what it was, then yea, take him out. Why waste time and money with a trial when vigilante justice can have the matter cleared up straight away? Where is your sense of vigilantism? (is that even a word? lol and if it is a word, I'm sure I spelled it wrong... but oh wells).