Do you think that when you morph a person you get some of their personality traits? Cassie mentioned wanting to make bad decisions as Rachel, but I'm pretty sure she was kidding. Tobias mentioned Taylor being gentle, but idk if that was a Taylor trait or a human thing, like Andalites being optimistic.
I think the long answer to this has comprised literally millions of volumes of text over the entirety of human history, all debating how much we’re who we are due to genes and how much due to our personal histories. And of course the answer is not additive but multiplicative: a genetic tendency to find it easy to learn grammar times early access to a computer equals a very skilled coder, genes that make you tall times encouragement to be athletic means NBA, etc.
But the short answer from Animorphs seems to be that the personality of the morph makes suggestions, that the morpher’s own mind can then listen to or set aside as the morpher wants. So Cassie in Rachel-morph feels flashes of anger, but doesn’t act on those feelings that we see, and Tobias in Taylor-morph feels a desire to be gentle but spends most of that scene acting about as cruel as we ever see him be. My read: the morph’s personality determines your impulses and automatic responses, but the morpher’s original personality determines your slower controlled thoughts and self-reflection.
So it’s like the classic saying “it’s healthy to be angry sometimes; it’s not healthy to act on anger right away.” IMHO a morph’s mind determines how much anger you have, whereas the morpher’s original mind determines what the person does with that anger. Or fear, or sadness, or joy, or empathy. Obviously the morpher’s actions will be informed by that first emotional response, but they won’t be dictated by that response.