I guess it's just really hard to let things go.  It's sad to hold on to anger and resentment for so long.

But, to be honest... I won't spend any time with my DH's ex either... I have no interest being around someone who tried to be so disruptive to my household... and hurt him and his kids.  It's not like I actively go around and think about her.. or anything.. but if a similar situation to what you described came up.. I would probably be walking around the block too..lol.

In my view, it's what the basis of the anger is over. In our case, BM is extremely bitter (and vengeful) because she is a very controlling person and lost that control when we married, then acted out like a full blown borderline/narcissist. She went all out on parental alienation as a punishment and then went to the effort (as if we care) to slander him/us to her crazy tight-knit fanatical community. These days, she goes out of her way to avoid DH when he visits SD (fine), but she will be in the same room with him (and me) at events (probably to enjoy her public torture of us...lol). I have asked many, many times why some people hold onto bitterness for a lifetime. I try to be very conscious of this because I myself have some lingering resentments. I do not think I ACT on them, though. Leaving a space because someone else is showing up is an act. 

Now, to me, avoiding is one thing . . . staying at one end of a large room at a gathering, etc. To leave a home and walk around the block seems like more of a message being sent. Whether it's a valid message or not is for the recipients to decide, I guess. For one person, that could be a childish, stupid move; for another person, self preservation. One would hope after 33 years, "feelings" wouldn't be right there on the surface like firecrackers. 

I feel for you re: future visits and gatherings. This will be something to, uh, navigate. We are lucky in that we live a long way from the skids so we don't engage in all of that.