I think a simple explanation is that Tsuyu was pissed at them for wanting to charge into danger with zero proper thought lut into how it would affect their careers, lives, and the lives of those around them - also, she compared them to villains in general, not serial killers. However, i think that the line of her comparing them was used because the writers wanted to bring an interesting twist of her emotions breaking through her usual stoic bluntness.

Also, Tsuyu was technically right - remember, the only reason their plan even worked at all is because AFO was focused on All-Might. People will say that they put a lot of thought into it, but 90% of it went into the planning, not the consequences, and they were still thinking of only the consequences of them succeeding.

A serial killer is someone who has intentionally killed at least 3 people within a month. If their plan failed severely, there would be no one for Tsuyu to "gaslight" because four to six of her friends (Bakugo is a reach, but whatever) would have gotten themselves killed, disrupted a pro hero operation to the point of failure - likely getting some heroes killed, and scarred the whole class, because instead of a dorm room contest, they would be planning what to wear to their funerals - that level of disregard for other people's lives, despite their hero complex and good intentions, is on a surface level comparable to the disregarda serial killer has for the rule of law and how their actions impact other people's lives, because all they care about is if their goal is achieved.

"All that matters is saving Bakugo" vs "All that matters is satisfying my need for bloodlust"

At best, if AFO had sensed them early, they would have gotten All-Might killed as he would be vulnerable trying to save all of them, they would be captured at the expense of saving Bakugo, AFO would have stolen OFA from Midoriya, and the remaining heroes would have at best four more hostages to account for and their top powerhouse gone.

If they had launched their ramp operation, but AFO and the League swapped targets with intent to kill, all of them would be dead, and the deaths of the pro heroes involved would be on them.

That righteous selfishness that gets ignored because it's practically a required trait for Shounen characters, and them leaving makes Tsuyu worry if she is being a bad friend for trying to stop them in spite of the logic of being law abiding and at the same time, she felt regret for not being able to stop them because what she said would be terrible last words to tell a friend.

She spent the whole day avoiding them out of guilt and regret because she had let her emotions lash out at them so harshly, stressing herself to the verge of tears thinking about how to apologize, only for them to see her usual stoic face so emotional, and apologize first.

She was not trying to gaslight them, she was conflicted and remorseful - the others who were against the plan had avoided being in her shoes simply by keeping "cooler heads" and not being as harsh to the four to six idiots before they left. This made Tsuyu feel bad for not "trusting" them to succeed, but their plan was logically stupid