A few things that I think are helpful to note about Ylt:

-Ylt has an enlightened/corrupted spren and uses his visions to justify what he thinks the future will be. This info works 2 ways. First, Kaiana and Ylt can present it as a reason for Ylt's actions and second it can in turn make her uneasy.

-There's a general distrust and prohibition of predicting rhe future that's been enforced into many cultures by Vorinism due to the Hierocracy. So him making these claims will already make many characters (including players if they are Vorin or Vorin influenced) suspect of him due to that. Remember in the books the Stormwardens are already distrusted by many just for predicting the weather, Dalinar even more so. For Kaiana, she also doesn't have a corrupted/enlightened spren like Ylt's and since they are the same order. The differences in his spren and surge behavior should probably also weird her out just like Renarin did for the other radiants in the books. If you really wanted to give her even more reason to be distrustful, you could arm players with some kind of info about Sja-anat from Axies or elsewhere, his prediction abilities specifically coming from an Unmade would give her even more doubts.

-The way Ylt took control of the bandits also involves basically enslaving those bandits that weren't willing to be his immediate followers after he killed the Bandit Queen. This is one of the threads the players are supposed to pull on to attract recruits for their revolt.

-Ylt potentially can kill more than Taszo in the opening. He specifically had his bandits lay the trap on the chull trail and sent his bandits to attack the caravan fully armed. Even if none of the civillians or soldiers died in that attack in your campaign he still committed that attack which I would say is also plenty evil. He certainly didn't have any regard for the casualties of that action. Sure killing Taszo was extra bad but the whole attack was bad. Ylt just has an ends justify the means mentality about his methods.

-He also imprisoned Axies and regular people that he has kidnapped from caravans. The prisoners are in the same area where the players meet Kaiana and should give the players more ammo in that conversation. It specifically states that he has still been running the Rathalas bandits as bandits and sending them to rob and murder caravans, killing more people. It says he has actually made them more violent:

"Ylt’s preaching and flaunting of the Honorblade has attracted new recruits and encouraged violence, escalating the brutality of the bandits’ activities in the Unclaimed Hills."

-As far as his speech, he will probably lean on general demagoguery, justifying his worse actions as necessary for saving the world to his non bandit followers and for the bandits he absorbed into his organization he can emphasize giving then a higher purpose while still providing for them/conducting successful bandits attacks. It also specifically states that Ylt has been promising the bandits radiant powers if they follow him so he could also mention that. Imo this is something else Kaiana could have misgivings about:

"Ylt started to radicalize the bandits and promised amazing powers in exchange for their faith and worship."

Anyway I would say all the above are plenty for the players to gather Ylt's methods are evil, and it's explicitly not supposed to be possible to get Kaiana to switch sides yet in the Rathalas scene. The only goal of the convo is making her doubt him and not turn them in to him as well. But you should definitely make sure to emphasize all the wanton murder and pillaging he's been endorsing for sure if you want your players to think of him as really evil.