>>723130238 (OP)
He was captured by Eldar once and the second company's former captain, Trajan, was killed in an ambush while attempting to rescue him. Titus took command and ended up rescuing Leandros himself using non-codex compliant strategy. It wasn't a problem for Leandros since then until the events of the first game.
Leandros could very well have been killed in what was, by all accounts, an incredibly reckless insertion strategy, and Titus' seeming indifference to whether or not Leandros survived planted the first seed of doubt in him. For space marines, and Ultramarines in particular, honour and brotherhood are very important. For Titus to dismiss his concerns at every turn and then to somehow survive massive warp exposure from the power stone seemed to Leandros to be conclusive evidence of chaos corruption.
In the second game, it's been almost a century since then, not only could the Inquisition find no evidence of taint, the Inquisitor that initially took him into custody turned out to be tainted himself. Leandros is deeply invested in Titus being tainted, or he has to face the fact that he himself betrayed a mentor and a friend, and he just can't face that. There's even salt in the wound in that Calgar went out of his way to honour Titus in front of the second company and Leandros himself.
He recommended Titus for a Mortality: Absolute mission after SM2, which was the Secret Level episode. Even the present captain of the second company outright told Titus "There are those who would see you sent back to the Inquisition", and Leandros was clearly goading Gadriel into suspicion. Leandros wants Titus dead or proven a traitor. Ironically, Titus is literally being watched over by Big fucking E.
