But you say, of course he doesn’t care, Pinestar left to become a kittypet! That’s not what I meant. I meant Pinestar leaving was only a catalyst for other events that will cause Tigerstar’s ambitious to flare to what it was, but it has very little relevance to the crimes Tigerstar ultimately commits.
He hates his father for choosing the kittypet life, but it’s not what drives him to be the leader of ThunderClan at any cost. Even if Pinestar had stayed the leader, if all else had been the same, Tigerstar would have still ended up killing Redtail. It wasn’t even really Thistleclaw’s harsh training that brewed hate within him. Rather, it was his loyalty to Thistleclaw and three essential occurrences which drove him to commit the crimes he did. Now, to be far, this is really only a theory, but I believe I have a lot of evidence to back it up so… here it goes.
It all starts with Redtail being chosen as deputy. Thistleclaw is said to be furious at Bluestar’s decision, and likely shared his anger with his equally upset former apprentice. It is clear that Thistleclaw will never have the chance to be leader. Not long afterwards, he is killed by a RiverClan patrol. It is not stated which cat killed him, but what if it were Oakheart? Oakheart and Thistleclaw did clash once before in Bluestar’s Prophecy so it could be Thistleclaw was trying to finish what he started.
Let’s say Tigerclaw knew the identity of his mentor’s killer. He finally has him at his mercy at the start of Into the Wild, but he has to save Mousefur instead. Then Redtail orders him to retreat from the fight. Later, Redtail accidentally kills Oakheart. Tigerclaw will never get his revenge, so he takes indirect vengeance on the one he has had a grudge on to start with, Redtail.
This theory makes even more sense when you consider he later kills Brindleface and Stonefur, Redtail’s mate and Oakheart’s son. It would also explain why Tigerstar wasn’t at all boastful when he met Pinestar at his leader ceremony. Tigerstar was never trying to prove anything to himself or anyone else. It was all a plot of revenge and karma for him.
Edit: His dislike of Oakheart and RiverClan could also explain why he allied with them, only to start killing their warriors in the Darkest Hour and why he seems far less fond of Hawkfrost than Brambleclaw even though, according Erin, he loves Sasha more.