Disagreement: When at least two people come together to discuss opposing opinions.
Bullying: When two or more people come together to dominate one or more people in a negative light. Examples: Starting false rumors, making fun of someone, teasing, exclusion of a party, etc. (I hope you get what I mean here).
When I think about these two words, I think disagreement can lead to bullying. But that is not always the case. People could respectively disagree on something and that is that. But people can disagree on something and it can cause conflict.
I don't know what you mean on drawing the line but when both parties are responsible and respectful to one another on a disagreement, that is fine by me. But if a disagreement leads to negative results like intimidation, bullying, harassment, etc., then it becomes a problem.
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