Reading through the books more clearly I am kinda mad.
1.) Leafpaw didn't trust Hawkfrost because he pinned down her friend who CROSSED THE BORDER. And wouldn't let her go/Threatened to take her to camp which is something any Riverclan cat defending their territory would do, to be honest.
2.) Leafpaw found out that Hawkfrost wanted to move in and take territory from Thunderclan while they were weak. Let me explain why this is no reason to think a cat is evil. Reason one, cats take territory from other clans all the time, it is not an evil thing to do, Riverclan and Thunderclan especially are always fighting over sunning rocks. The second reason, if I remember correctly, pretty much only Bluestar was against taking Windclan territory when they had yet to return after Brokenstar, all the other clans were fine with only having three clans and more territory between them, may I also add Skyclan? It is completely justifiable, in my opinion, that Hawkfrost wished to get more territory for his clan when the others were weak and easy to take from.
3.) He is a really smart cat, who only does what he thinks is best for the most part. He joined Mudclaw because he truly believed that he was meant to be the true leader, lots of cats believed that. Brambleclaw himself thought he wouldn't be accepted by Starclan.
This was a really rushed post and I'm too tired to proofread, sorry.