Funny, Morty was actually one of the less-likely cases. Most of the others fell, he got pushed. Trapped in the warp, his Marines dying around him until he swore himself to Nurgle just to make it stop. Fulgrim was also pretty forcibly corrupted by the Laer blade. Most time-bomby I would say were Angron, due to the Butcher's Nails, Lorgar, due to him coming from a culture that worshipped Chaos and him trying to worship Empy, and Curze, because Curze was batshit insane from the moment he clawed his way to his planet's surface. The rest could've probably gone either way.
Personally, my fave's Vulcan, but it's gotta be undestood that it's all relative. He's a humanitarian by Primarch standards, but that just means he feels bad about causing all those innocent deaths. The Khan is similarly bro-tier, as Primarchs go.
And honestly, leaving the Primarchs laying around probably wouldn't have worked out better, long-term. Roboute already had a stellar empire of 500 worlds. Dorn apparently hails from another star-empire, though we know basically nothing about it. Most of the others would've gotten out there eventually, and then you've got 21 (can't forget Omegon) interstellar warlords, but now they're all fighting each other rather than moving in the same direction. The mistake was making them to begin with, imo.