Uldren, despite being manipulated, "Taken" or corrupted by Darkness, still chose to do what he did. Taken aren't forced into being Taken. To become Taken you're coerced into it. Your biggest insecurities are then reshaped into your greatest "strength". Anyway, Uldren still chose to kill Cayde. But that's not it, he didn't just kill Cayde. He slaughtered his own people, until they eventually scattered. He killed people who had families. He killed brothers, sisters, mothers, fathers, and maybe even children. Do you understand now why Mara didn't really care he was dead? Why Petra was adamant about killing him even though she locked him away the first time? "You have no idea what he's done!"