Sarek's advice in the pilot was to murder as many Klingon's as possible, or they will never respect you.
It's way more likely that Sarek would have went there to kill Klingons than it ever would have been than Katrina was bring back scalps.
It's a little out of character.
Well...perhaps. But, as has already been commented upon, he is not exactly the best judge of situational awareness. Plus--and this is interesting since we're talking about Vulcans here--there's a certain degree of hubris involved, perhaps even what we would consider narcissism.
But there's another thought. There's a concept in behavioural economics called prospect theory. Long story short, if you're already digging a hole with a mammoth case of sunk-cost bias, you would think that you would stop. But that's not the case. If you are in the domain of losses, you double-down. You try your best to keep on pressing even though you, rationally speaking,
know the odds of your success are slim and dwindling. It's why the house always wins in gambling. Gamblers who are not sufficiently self-aware put themselves in the domain of loss and the get gripped by sunk-cost bias, shoveling more and more effort into a clearly lost cause.
So, what has been going on in Sarek's life, such that he might have felt compelled to undertake what any other sentient being could have easily predicted was a trap?
Item 1: Michael's mutiny.
Item 2: The backlash from that--after all, I am sure
many Vulcans were more than willing to remind Sarek of his apparent failure to teach the human how to be a proper Vulcan--could reasonably be inferred to have caused internal pressures, negative affects, and otherwise deleteriously affected his decision-making calculus.
By the time the "negotiators" had gotten in touch with him, he was primed for making a disastrous decision. And so he did.