Cathy: You’re a cynic!
Steed: And you’re an idealist!
Cathy regularly accuses Steed of being a cynic - he distrusts anyone who does not have an apparent angle. The show does not always prove him right, either, although there are moments when his cynicism is probably justified. In this case, though, he actually tries to help the man trying to kill him. So perhaps Cathy is bringing out his basic humanity, or at least pointing out that maybe he needs to start tempering his attitude.
He also practically throws a fit at her at one point and tries to beat up a clown. There’s that.
Short URL for this post: http://tmblr.co/ZLO6KtglVBVq