I was really shocked that Lorne killed Lindsay because Lorne is not really a violent person or a killer. But I think Angel made the right decision because Lindsay only cares about power which means he will probably betray Angel and the team. So Angel couldn't take the risk of having Lindsay alive.
I don't believe that Angel had made the right decision. All he did was coerce Lorne into committing cold-blooded murder. Neither Angel or Lorne had an excuse. I don't buy that crap about murdering someone because they might pose a future threat. That bullshit and a lousy excuse to indulge in violence. It's one thing for Angel to be aware of Lindsay as a future threat. He should have waited until Lindsay actually became a threat, instead of ordering Lorne to commit murder.
I get so tired of hearing excuses for Angel's questionable actions.