In both Kingsman: The Golden Circle and Men In Black II, the mentors (and the very bankable actors playing them) return from death/retirement. Hooray, we liked those guys! Except that bringing them back ruins the character arcs for the guys we're actually supposed to root for. Instead of growing into capable badasses, the main characters get booted back to new guy status, sidekicks running to their hero-daddies to get them out of trouble.
By the end of Men In Black, Agent J was no longer a rookie agent. He had his own suit and shades (a big deal in the MIB universe), but most importantly, he now had a rookie partner of his own to show the ropes. But in MIB2, J no longer has any of these, and the whole story revolves around him running back to Tommy Lee Jones to get him out of a galactic pickle. The same happens to Eggsy in Kingsman 2. He's supposed to have become a full agent and bona fide world-saver, yet he still needs his formerly deceased mentor Harry (a guy who had most of his frontal lobe blown out), to come back and solve the mystery for him.
20th Century Fox
This also ruins the mentors themselves, completely undercutting the emotional impact of their departures. Kingsman: The Golden Circle even has the gall to pull this stunt twice, killing off new mentor Merlin solely to make room for Harry again. Nice try, movie, but you've by now established that A) getting shot in the head does nothing, and B) nothing in the universe matters at all. So instead of feeling anything about the death of an important character, we'll wait until Kingsman 3, when the writers stitch him back together, Frankenstein-style.