The Lions led by 17 points during the second quarter but fell apart to lose by 45 in arguably their most disappointing loss of a difficult season.
They were trounced in all facets, losing the disposal count (392-312), clearances (52-30), inside 50s (61-41), contested possessions (132-122) and tackles (73-47).
A flat Leppitsch said his young team would have ups and downs, but he didn't have to accept them – and he sure wasn't on Saturday night.
He was particularly critical of the defence from stoppages in the Giants' forward 50, which they conceded five goals from.
"That's a pretty general sign that your accountability and ability to be switched on is not there, and I really felt that was the case," Leppitsch said.
"It's actually been a concern for a few months. It's one of the things we're trying to work on, is to respect that defensive side of the game and it just fell apart."
Leppitsch was full of praise for the Giants' intensity and said it was a harsh lesson for his team.
He said being last for much of the season, teams had often been "nice" to the Lions, and said they did not handle the hungry Giants well at all.
"I could tell from the first minute," he said.
"The more I get to know this group, five minutes in you think "wow, what do we do?"
"It took until half way through the third quarter before I think we ran out of options of things we could structurally try to get them back into the game … a bit of trying to find a way to get them sparked, but it just wasn't there."