Where is the kingslayer’s hideout

This is located in La Valette's castle, very early in the prologue (The King's Will, Part 2 - Barricade).

The king orders you to capture and use the ballista. You drop down the scaffolding on the wall and turn left to see the ballista. You'll have to fight several foes to reach it. Once you've beaten them, continue past the ballista, toward the closed gate. Poor Altair is on your right, slightly hidden by the castle wall.

Upon finding him, you'll receive a bonus ability, assassin, which grants you +25% damage when attacking an enemy from behind.

You can use the Cat potion to easily see the blood trail in the forest. There are at least three trails to follow, all starting near the gate between Lobinden and Flotsam. By keeping (mostly) to your right, you will end at human corpses. But if you try to keep to your left, Geralt will mumble something about "he is getting weaker, can't be far now." That trail also ends in someone else's body. But from there turn around and keep to your left again. Cedric is not far from there.