I really like how sometimes when you get close to those explosive enemies, this red ring appears so you know you need to get out of the way or you'll die. It would be even more fun if they always did that. Or at least there was some way to tell if you'd get a warning. Approaching them could either be a way to bait them into exploding without having to deal tons of damage to them, or it can be instant death, and if there's a way to tell which I haven't found it.
Basically, they have 2 ranges. First is the blast radius, and inside that, is the radius where you trigger the self-destruct timer. If you're inside the blast radius, but outside the self destruct range, killing them deals damage to you, without any prior warning.
I agree that it's harsh, but there is a mechanical reason why they sometimes kill you without lighting up.