Because lonely people have nothing left to live for and should just die anyway right? I see this trope alot where some dude is about to kill someone but then they look in their wallet or some shit, see that they have a spouse and kids and think, “oh this person has a family, that automatically gives their life any semblance of meaning that it would completely lack otherwise, better not kill them.” I get that it sometimes works as a device to show that the person about to do the deed could perhaps have some family ties of their own, but why is it never the other way ya know? Like a dude is about to kill someone but then finds out that they are actually alone and spares them because of that. Something to spice things up maybe.