who will live longer the 120kg person or the 60kg person?

It depends on genetics, lifestyle, their height & loads of other factors.

The 120kg may be a male professional rugby player, 6'8, healthy lifestyle & great genetics.

On the other hand, it could be a 5'3 woman, lives off junk food & has cancer flying around the family like there's no tomorrow!

Weight becomes a serious factor in life expectancy at extremes. It is not a particularly good indicator of health and fitness. The weights given could both be very healthy or very ill.

The one without a bullet hole in the head.

The 60kg person, definitely. I think 60kg is only ever so slightly in too much weight. But if a person is 120kg... they are twice as fat and heavy. Not healthy enough to live long.

Depends on who fires first - doesn't it.