It may not be illegal but I bet you'll end up getting sued and potentially losing. While I understand that if, for any reason, you have to shoot someone, you'll probably end up in court. Firearms are such a hot button topic that the dems would be licking their chops if something like this happened, IMO. I guess that's where it becomes a personal choice. Proceed as you see fit and be ready for any repercussions. I wouldn't convict anyone for the above scenario but there's a lot of people that probably would. Your life wasn't in danger until YOU decided to enter the home. In the big picture, it's probably better to just stay outside and let the police do their thing. For safety's sake, liability's sake, etc.
Ronin, it definitely was a completely loaded question. That's the picture that could be painted should you get sued. Guarantee the prosecution would paint you as a trigger happy vigilante that was just waiting for a reason. After all, why else would anyone (that's not a professionally trained law enforcement officer) go into a house to check it out while the cops were on their way (outside of the situation where a human was in danger inside the home)? See what I'm getting at?
I think once someone breaks into your house, they should give up any/all rights but that's not how it always works these days, unfortunately.