I weep for what the world lost in his genius, but maybe it's for the best. But seeing he was perfectly willing to destroy innocent and reformable groups when it wasn't necessary. I was willing to follow the guy because he was an interesting combination of crazy prepared and only as evil/manipulative as necesary. A faction that bloodied the nose of the NCR. And even got on extremely good terms with the BoS. Reformed two of the four families to keep them this side of the straight and narrow. I mean, I turned the Boomers into an asset.
For me, it was the combo of finding out what he did to the Vault under Vegas and when House ordered me to wipe out the Brotherhood of Steel bunker.