Well I gave it some thought, loaded it up, and let him live. Yes he is a son of a b**** but by letting Roche kill him, he would be a kingslayer and as bad as Lethos.
Also there is the matter of the state of Kaedwini. Temeria was in a bad state after the death of Foltest.

Live by the sword, die by the sword
After he backstabbed his ally Aedwin with his "brotherly help" mission and divided it with Nilfgaard 5 years ago, and 2 years later tried his land grab again it was easy for me let him die

The swine dies every time I do Roche path, I'm only pissed I don't get to kill him myself.
I hate how he thinks he's above the law, every time I get to humble people like him in RPGs I do it.
If you let him live he becomes the most powerful ruler in the North, would you want someone like him ruling the North?

I play with Roche and dont let him kill the King Henselt.

Why?

Witchers and spies are not murderes or assassins of kings, our enemies. And also I always thinking in the enormous consequences of this regicide in the future.

On the one hand, Henselt is a swine and deserves to die.

On the other hand, nobody, not even Roche, can handle the burden of having killed him. (This is borne out in Act III; listen to the banter between Geralt and Roche as they approach Loc Muinne.)

So my Geralt persuades Roche to let him live, so that Roche will not have to live with Henselt's blood on his conscience.

really? the walkthroughs say that this is one of the decisions that affect what ending you will reach

It has no effect on actions you take in chapter 3. It does change the political picture presented at the conference. It may affect saves that are exported to Witcher 3; we don't know yet.