Danny Ings' double and Jan Bednarek's first of the season saw off a a Villa side low on confidence having failed to win in the league since November.
John McGinn hobbled off the pitch after a serious ankle problem meant his afternoon was prematurely cut short.
Dean Smith spoke to Birmingham Live about the disruption caused to his team after McGinn's injury.
"We've got El Ghazi marking their centre-half when he shouldn't be.
"John McGinn (should have been) but, after he went off, I believe it should have been Douglas. The message never got on. El Ghazi ends up marking him and the lad scores a free header."
After conceding a second goal before half-time, it was an uphill battle for Villa there on.
"Players have to take responsibility at times, and I have to take responsibility as well. We've conceded seven from the last two home games.
"We were very poor defensively against Leicester and, today, they were ore individual mistakes that's for sure.
"First one, Bjorn gets done over the top. The second one, it's a corner and a free header and the third one, Fred must get closer to the lad and it ricocheted off Marvelous. There's certainly elements we need to get better at defensively."
Smith spoke to the Express & Star about leaking too many goals recently.
"We've conceded seven from the last two home games. We were very poor defensively against Leicester and, today, they were ore individual mistakes that's for sure.
"Southampton made it hard, competed and won the second balls. That's what disappointed me, we never showed enough commitment to win the second balls."
Above all, Villa boss Smith was disappointed with his sides' lack of application against a fellow relegation struggler.
"If you give away a three-goal lead in any league, it’s tough, but in the Premier League, it’s tougher. They won the second balls, and that’s what disappointed me today.
"We lose a race against Long and we give away a free header. We started quite well but after the first goal, you see heads drop."