If we were in a European final this week I wouldn't expect United fans to be cheering us on nor would I want them to. What happened this week was dreadful and if any United fans attended on Monday night I sincerely hope they are all OK and survived the attack. To any United fan who didn't then RIP and deepest condolences to their families.
However this is football rivalry which is a completely unrelated topic. It's not being disrespectful in any way to keep that rivalry going whilst they are playing. So come on Ajax!