If Aegon had viewed Liliana as his mother, then perhaps he might have resented her for what she did. Even if it was under stress and emotional instability, she did abandon him at the moment he needed her the most.
But that was the point.
He never saw her or treated her as his mother. Some might call him a pushover, but he really didn't want anything negative in his life unless he couldn't help it.
Hating Liliana or his biological father would have done nothing but add negativity to his life. So he never bothered with it.
He was grateful to them for bringing him into this world, but then abandoning him severed that bond between them. Not to mention they were absent all his life, so really they mattered little to him.
So, when he heard everything that Liliana went through, he only felt pity for her. This was what happened when one chose something they couldn't undo.
