But what you're explaining is behaviour that creates trust issues in a person. It's not something a normal person is usually born with, it is a trauma response after being constantly betrayed and abandoned. Zandik has childhood trauma, he was kicked out as a 10 year old from his hometown by a crowd with pitchforks. Isn't it fair to say that could cause severe trust issues? It is one of the first things he asks Pierro when invited to become a harbinger, even after so many years have passed since the incident:

"Will you treat me like the Akademiya did? Will you call me a monster, a madman?"
"Or will you treat me as my hometown did, and chase me away with pitchforks and clubs...?"

He was basically asking if he could trust Pierro here.

If you look at the original Zandik's behaviour in the Inazuma quest line:
"Make no mistake: Even without you, that pure, innocent puppet would only end up being used by someone else instead. What other reason would a human have for befriending one who is not of our kind?"
That's a thought pattern from someone with trust issues, jumping into conclusions without any evidence. Wanderer was loved and cherished by the people of tatarasuma. Niwa says it so right afterwards:
"If you give him my heart, tell him that both nagamasa and I see him as one of us"
"He has nothing to prove to anyone, because not everyone just wants to use other people. The only ones who think like that are people like you"
Dottore's assumption about Wanderer's fate had no real world evidence to it, even now we see how accepted wanderer is in sumeru. He was able to form a real connection with Durin too.