In the news

I think the motives are pretty obvious.

I assume in case he had a bomb?

Why didn't they shoot to injure/disarm?

Maybe they did but he died anyway, maybe the shot they took was the only one available, maybe it was scary and high pressured and they did what they could, maybe the shooter thought 'fuck him, he's going down' and put one between his eyes, maybe it's all a massive conspiracy by the lizard ppl.

what do you think his motives might have been?

In a complex and fast moving situation, you don't know that they didn't attempt this. The narrative and emerging images clearly show an attempt to treat and resuscitate the attacker after he was shot. You don't do that if you've just attempted a kill shot.

If you've ever fired either a Glock 9mm or a submachine gun you will know how difficult it is to hit a small target, especially one that is moving, presumably fast - especially if you don't have time to brace against the recoil. Aiming for a leg is less likely to succeed than firing into the torso, and therefore more dangerous to both the shooter and any unfortunate passers-by behind the target.
Looking at the pictures of him on the stretcher it appears he was shot in the stomach, which indicates the officer went for the largest target.

I could be wrong here, but generally when shooting a suspect the police will aim for the body mass rather than say a leg or arm as there is less likely hood of collateral damage from a stray bullet that has gone in and out of someones thigh, training kicks in.