I appreciate the long message! It's cool to hear the thought process behind it. I honestly think the way you have it setup is pretty good. Even if someone initially avoids killing the guys, they would likely notice the life bar and through trail and error figure out what causes it to decrease.
"Show don't tell" is one of my favorite aspects of game designs. When done right the player will feel real ownership over solving a puzzle or figuring out a mechanic. One of the most satisfying feelings a video game can convey imo