... Harry is blinded by love...
I don't think he is; not really.
... Thomas the human partbis a good guy. And harry sees only that part. He cant imagine Thomas the vampire or he cant allow himself to see that part ...
Remember: he has
Seen the Hunger Demon inside Thomas.
Seen it in a way he cannot un-See.
Harry has seen Thomas lose control (and knows how to deal with Thomas losing control). And when he knows Thomas is stressed/injured & close to the edge, he takes extra care. We saw him with Susan, feeding her a bit of blood that she needed, but then punching her in the face to stop her feeding.
Harry just
chooses to believe that Thomas is
mostly going to continue to be worthy of Harry's trust.
And let's face it: most people are occasionally un-trustworthy... this "problem" isn't unique to Thomas! About 90% of American adults have drivers' licenses, and when behind the wheel they are one moment of rage (or simple inattention) away from hitting someone harder than Thomas ever could. Yet we drive public highways & walk down sidewalks
by the millions, daily...
Everybody human is fallible. We can all do tremendous damage, in a critical moment, if we do fail. Hell,
Harry himself can do more damage than Thomas could! Harry's entirely cognizant of this. We need to extend trust, if we're to be trustworthy ourselves: if we treat everyone else, at all times, as if they were on the verge of being their worst selves, that'd be a terrible way to act and to live.