Questions about I am Legend?

Who set the trap and put the dummy in the puddle, Neville or Anna or the zombies?

Why did he kill his dog and not just lock him up til he found a cure?

At the end why didnt he throw the grenade right before they broke the glass then run in the secret hole?

I'm pretty sure the zombies set the trap at night because they saw how he set the trap and they needed food. I think he killed his dog because he didn't want to see her turn into a zombie because it would have hurt him more to see her like that. He didn't want her to have to suffer as a zombie. As for the secret hole, if he didn't die for the cause it would have had a different effect on the movie, but I wish he lived :(

The lead zombie set the trap to get back at smith b/c he took the female zombie. He had to kill the dog, he couldnt hold it still to get it into a cave i think. The third one I was thinking the same exact thing because he did not have to kill himself like that... to me he could have just thrown it and ducked into the cavern.

1) The Vampires. They weren't zombies. Zombies don't burn in sunlight, at least in any movie I saw.
2) He had given up hope. He had been trying forever to find a cure and he didn't think he was going to find one.
3) It's a movie. He had to be a "tragic hero".