Are there silent kills?

[Urinietszche]

I tried to assassinate slavers near NCR with all my dynamite leftovers, so I put it into everyone's inventory from stealth (assuming they didn't see) but when it exploded it did just 30hp damage and made them hostile towards me (how could they know???) so is there a way to be ninja killer, assassinating enemy from stealth without engaging whole level of enemies?

[Mennan]

Largely no, there is largely no stealth kills in this game. There's a few scripted stealth kill events(typically with assassination missions) but not many.

Moment you kill one person everyone in that faction will engage you in combat immediately on sight.
You can use stealth to decrease their aggro radius but that's about it

People say you can sneak kill with dynamite if it actually kills them but largely this just invites the crash bug when the dynamite goes off.

The only way for guranteed sneak kills is to use super-stims.
If you super stim a target at full HP then when the -HP modifier comes in it will accumulate enough damage to kill them. This will cause no aggro but it typically takes alot of super stims in most cases.

[Urinietszche]

I got very disappointed with dynamite sneak kill, because I put it inside man's pocked and it did him 30hp damage from his 90hp pool, and he was wearing just metal armor, nothing fancy. And there's not much free dynamite in the game anyway. BUT WHEN EXPLODES ON ME BECAUSE OF SKILL CHECK FAILED I GET HURT ALOT.

[Mennan]

Pretty sure the damage they take is effected by the DT/DR so same thing will happen to you. If you have a piece of armor with more DT/DR to explosives then you would take less damage.

Edit: on the bright side if you walk out of the cell to set off the bomb and back in the cell it looks like they wont be hostile.

So you could always take the effort to plant bomb walk out of cell then walk back in to plant more bombs if they're not dead.

[The Chosen Chad]

You can stealth kill, with high enough sneak you can kill a critter without aggroing everyone or getting the local law on your ass. I personally killed Metzger with point blank 10mm SMG Burst with a relatively high sneak character (and using Jet and Beer on every slaver to decrease their Perception) without having anyone noticing, they even talked to me after that like everything was normal.
You just need to kill your opponent in a single turn.

Alternatively you can you use superstims to kill off non hostile critters.
In the Nevada mod, you use poison to directly kill them too!

[Gnord]

The mutagen serum is also a stealth kill, but it's only 1 shot and it only works on super mutants.