If a god does exist and he showed up in front of me with unassailable proof that he is real, and he asked me to kill my son, I'd tell him he may be all powerful but he doesn't have the ability to lay a guilt trip on me. I'd tell him to f**k off so fast I wouldn't have time to include the asterisks. Sure, they might be my last two words, but at least I'd die satisfied that I wasn't a tool for a power-hungry despot.
The why's of his request would be irrelevant. The most he could come up with would be a litany of excuses, and I've learned from a couple of women in my life not to pay attention to any of those.
If I instead decided to do his dirty work for him and then earned a trip to heaven, it would suck if I had to spend an eternity with the dude that had me to off my kid.
Game playing is something humans do, so they tossed stories like that into the bible in an effort to impress underlings with the supposed power of their "god". Bribery and threats don't work on me that easily. Even the real, human ones.