It makes you a horrible person. (Nah, just kidding. Choices you make in video games do not reflect on your true personal character, I don't think.)

I beat the game yesterday. The gameplay itself of the ending was pretty awesome, and a bit more challenging than the ending of Fallout 3. The cinematic stuff after the ending... left something to be desired, kinda like with Fallout 3. I was playing a little while today to try to get the NCR ending too... but then the game froze up again, and when I went to load it again, it kept stopping at 3% when loading the trophy information, so I took it as a sign to stop. btw...

SPOILER!!:
Considering they talked up the whole "player being able to walk through the game playing the diplomat in all situations," you'd think they could come up with a situation in which Mr. House continues to live, secretly. The NPC's themselves walk around saying "How can you be sure he's dead? Nobody's seen him in years," or whatever. I mean, you don't even get to tell him you were sent to kill him until you've already done the damage anyway.

Also, you're only partially right about not having to destroy the BoS; with the NCR, you can negotiate, but Mr. House wants them dead, no exceptions. Mr. House is kind of a bastard. I'm kind of surprised you get bad karma for killing him. I guess he is the "last hope for humanity..."
Anyhoo...

Wuv, Yer Mom