ROTJ and KOTOR shows otherwise. Quite literally, love saved both Luke and Vader, and even in Dark Empire, Leia's love for Luke dragged him out of the darkness. Meanwhile, the Jedi treating love as the enemy leads to complications that causes people to fall to the Dark Side, especially when these Jedi have no love for the Republic, so the moment they realize that remaining as Republic slaves stunts their metaphysical growth, they leave.
The Jal Shey and Zeison Sha didn't fall to the Dark Side. Most of the evil Dark Jedi we get come from the standard Light Side Jddi Order that treats love as the enemy. That includes the original Dark Jedi who founded the Sith Empire when they landed on Korriban, Exar Kun and Ulic Qel-Droma, Revan and Malak, Phanius and his 50 Jedi Knights, Anakin Skywalker, and Count Dooku. The worst Darth Traya did was train some Sith, but then they rejected her and she defected to the Jedi Exile who killed them all.
The Dark Jedi crashouts from the Light Side Jedi Order that treats love as the enemy caused way more damage than any Grey Jedi I know of. Not even Darth Caedus caused as much damage.