The Villa scoreline was dominated by John Carew's hat-trick and Keegan said: "I had no complaints about the first half at all. I said to the players we had built a good platform from which to go on and win a football match.
"Instead of going a goal down like we have been doing in most recent games, we went in front and handled everything Villa threw at us. I impressed at half-time on the players the importance of the opening 15 minutes of the second half and picking up where we left off.
"We were unlucky with Villa's first goal which took a deflection and even then Shay Given would have saved it if he hadn't slipped and pulled his groin. You want to see how the players were going to respond after that and that was the most disappointing aspect of today.
"Before you knew it, we were getting bullied, especially by Carew and Harewood, and they were basically doing what they wanted to do with us at times. That was the disappointing aspect and that is what I told the players. There were still some very good performances but who cares when you get beat 4-1.
"We got the lead today and still couldn't go on and win so you sort of run out of excuses in that department. When you look at the second half the positives are forgotten because of the negatives and we are very fragile at the moment."
Keegan is confident Newcastle, without a win in nine league games, will not be dragged into a relegation battle.
"We have got enough to be a Premier League club next season but you have still got to get to that 40 point mark no matter what type of club you are."
He said: "In the first half, Newcastle played some pretty football, deserved to be in the lead and we were second best.
"Maybe we were suffering a hangover from getting beat at Fulham and because of the internationals this week we didn't have a chance to discuss it until Friday night.
"But we got the goal just after half-time and it reignites the crowd, put us back in business and we were excellent after that."