Yes to the comma as the sentence stands, but I'm not sure that "Aside from" correctly expresses the relationship between the two statements. The dash is correct. "pessimistic thoughts" and "tends to" are correct in themselves.
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RommelHaving/harboring/with pessimistic thoughts like “Nobody cares about me!”, my friend Tony tends to conclude that all people around him are the same – selfish, indifferent, or unkind."Harboring pessimistic thoughts ..." would be my choice there. Yes, that works now.