Vidjya Game does state that you'll need to be able to see it, as otherwise you have no feedback on what's going on.
a mandatory compromise, i guess.
yeah, it's pretty unlikely by the time you Kill someone that enough Steam would build up (so a couple seconds) to cause them to explode.
and if you could pump enough Energy into them to do so:
1 - wow, this is an incredibly dangerous Weapon, just by being near the Apex you're going to be almost as dead as the dude in front of you.
2 - if you can generate and pump that much Energy, you can do it in a more efficient way

3 - would probably turn into cinders from spontaneous combustion before they explode anyways.
Vidjya Game rule of cool as ever. making Enemies explode is cool and being able to see what's going on is rather important.