"If she enthralls me again, I'll hurt people I care about," said with a very pointed look at Yuya. Strange doesn't have to say 'remember when I tried to murder you in Portland' to get the point across that's what he's thinking of.

There's a whole host of other factors: if he's enthralled again, she might go for his soul, the Ringmaster might not help, he might not see Arabella again. But those are all what-ifs and maybes sprung from Strange's paranoia, things the he knows he can't deal with and yet he worries about anyway.