Visser 3 is also very scared, and feeling grief and anger about all the deaths, though he's trying much harder and mostly succeeding at keeping that at bay.
Most of his thoughts are on the fact that Leareth is basically him, except a couple thousand years older and wiser, and correspondingly more wily and paranoid - he keeps noticing parallels, it's not just Seerow and Urtho, the entire way they think - the way they relate to their emotions, to their goals, to people, to the world - is similar. Except Leareth is better. Visser 3 still doesn't even have a hypothesis for why this is a thing but it's pretty undeniable.
Leareth lost. In his version of their story, whatever the hell that means, he didn't have the information he needed, he failed to de-escalate, and Urtho's superweapon nearly destroyed the world. Millions of people died, not billions, because Velgarth is smaller, but still. Leareth still regrets it. Leareth said instantly that he would have surrendered, even if it lost him his empire and everything he had built, because he can do math and that wasn't worth the risk of what ended up happening.
Five billion people.
And - Leareth's thoughts on the upside of trust, and what he's giving up by closing all those doors. Leareth's years of talking to Vanyel. Matirin wanted to persuade the Andalites to give the Yeerks morph, if they won.
He doesn't know that it's the right gamble, but he's in Leareth's head and he is, at this point, pretty convinced that Leareth knows what he's doing more than Mhalir does. He's had so much more time.