Never said we were perfect, but you'd be royally screwed if the Imperium didn't exist. You can exalt all your past deeds and glory, but the fact of the matter, Xeno, is that you're clinging to an idea that died a long time ago.
Well then we seem to have something in common with the Imperium of Man that clings on to the twisted teachings of your Emperor. Our teachings are as pure as they have always been but the moment the Imperium has a crisis its entire ideology. Without an Emperor there can be no Empire. It is only a matter of time before your "Imperium" goes the way of all men and turns on itself. Mankind likes to see themselves more civilised than the Orks but if there is an opportunity for man to seize power from a superior all it will take is a word or two and they will kill each other without remorse. Funny thing is Man will do the exact same the day after learning nothing apart from they must close themselves off from others to ensure no one can betray them again and everyone be they "Xeno" or even your own race they are under constant suspicion of treachery. The Eldar's days are numbered as are all, the Imperium's fate was sealed since its birth. When Eldrad warned the Imperium about the Horus Heresy as you call it he was betrayed and many Eldar died because of it. That was a clear sign that the Imperium cannot and should not be trusted. Other Eldar might see the Imperium as possibly trustworthy but I would rather live with Orks than see the Eldar stoop into an Alliance with the Imperium of Man.
I thought I smelled the taint of xenos. Welcome witch. Your existence is an affront to all that is pure; we will cleanse you with ignited promethium.
You know you don't need to cut me open... You could just ask rather than start cutting me and my tools apart...