I doubt that he would have reverted back to anything like Angelus, but he would probably have withdrawn from society, as he was in "Are You Now..." at best or reverted to the rat eating, alley dwelling, homeless Angel that he was before Whistler originally found him. Angel has always struck me as the kind of person who is by nature solitary, even reclusive. From the first time that we saw him in Sunnydale until he left to go to Los Angeles, he is always alone, with Buffy, or dropping in at the library with some tidbit of information, then leaving as soon as he delivered it. Angel was never the sort of person to hang out with friends at the local bar or have the gang over for hot dogs, beer, and a John Wayne movie marathon. As far as that goes, Angel was never shown as ever having any friends. He was on relatively civil terms with Giles and the Scoobies, but the only person in Sunnydale who he ever was on more than a nodding acquaintance with, was Buffy.
Doyle did in LA almost exactly the same thing that Buffy did in Sunnydale, without the "moment of perfect happiness" factor involved. He drug a reluctant Angel out from under his rock and forced him to associate with people on a regular basis. Without Doyle and Cordy, and, later, Wes and Gunn, Angel would in all liklihood have started out like Batman, lurking in the shadows, leaping out to strike a blow at the bad guys. But, unlike Batman, there would have been no Alfred, no Robin, not even a Bruce Wayne secret identity in which to withdraw from the fight and rest occasionally. Angel would always be Angel. But, very possibly, he wouldn't be the hero for very long.
Buffy, and his feelings for her, were the only real things that ever inspired Angel to he a hero or champion, to help the hopeless. As far as we know, from what flashbacks we were shown, he never went out of his way to fight evil before coming to Sunnydale, and he never would have come to Sunnydale if Whistler hadn't dragged him out of that alley and convinced him to help the new Slayer. Buffy was the spark that started the fire of Angel's heroism, and the flame that kept it burning. Without that, or a substitute for it, Angel's moment of heroism could not have lasted indefinitely. He almost certainly would not have become a creature of evil again, but it is highly likely that, without continued encouragement from a different source, primarily Doyle in the beginning, but just as surely Cordy and the others eventually, Angel would have given up the fight within the first year. He would never have reached his moment of epiphany, "If nothing that we do really matters, then the only thing that really matters is what we do," and without tyhat, he would never have developed the fortitude necessary to continue the battle on his own.
