This is delicate enough he should send it rather than say it, probably -
He's not angry with Leareth, obviously. It's hard to guess what exactly Melkor did but if it were him he'd just erase all Leareth's recollection of being in Arda, feed him all the same input that put him on the side of the Quendi from within Angband - and probably Melkor is at least that smart - and it wouldn't be better if Leareth wasn't the sort of person who did things.
He'd been angry at the time. Mostly scared, but - some amount angry. He'd been unsure if - if maybe Leareth had just decided that actually the most important thing, given his goals, was to have powerful allies - it felt a little off but not impossible, and there were Gates popping up all over Valinor to prove that Maitimo wasn't paranoid enough -
- and he'd thought about it and decided that even if that was what had happened, he would use Vanyel to kill Melkor and rescue Leareth, and then - what, right, hold him prisoner forever? For wanting to save his world more than he wanted to save Maitimo's? For being the kind of person he had to be to have landed in Arda in the first place instead of dying thousands of years ago, a person who played the cards in his hand, even if none of them were nice ones -
- it's not even an applicable line of reasoning right now, not having seen what Melkor did to Leareth's head, but - maybe it's a useful bound on things. If Leareth had spent this whole time devotedly working against them Maitimo would, well, firstly be confused that they'd still won and then he would work on making sure that he was on all of the best roads to accomplish Leareth's goals.
Probably the part where he held him prisoner would've hurt less, though.