Yeah, there's a problem with the earth's gravitational pull.This is heavy.
Not Doc's best line, but a decent one.
Absolutely not.So, he virtually killed his brothers so that two strangers would have a better life (at least financially)?
1. One of his siblings was female.
2. The two strangers' lives were improved on multiple axes.
3. Killing his siblings and giving two strangers a better life were two separate actions on his part. The former happened when he shoved George out of the way of that car. The latter when he manipulated George and Lorraine into happily ever after.
*~*~*
Another question, though. Once Marty had picked up the guitar and ensured that his parents would get their last dance, he had presumably locked things on course. George and Lorraine were going to kiss and realize they were going to be together forever, no further action on Marty's part required. So why didn't the photo return to normal right then?