Thief green skill is a heal, not life steal.
For the first question (before talking about the thief again), wee witch blue skill negates healing received (if you hit, as an example, a plant soul, she will not heal herself, but will be still healing the others). She negates X healing and does the same amount of damage.
Let's imagine a wee witch blue skill doing up to 5k damage and preventing 5k healing, and the enemy hero casting a 5k heal.
She will completely negate the heal and do that same amount of damage, effectively converting the heal into damage.
But things are not always so nice.
The thief case: imagine that thief with +500% improved healing, doing a 20k heal with her green.
Imagine your wee doing the same 5k damage and heal negation.
All happen at the same time: damage, negating part of the heal and the heal itself. At the end, if the healing is higher than the negated heal, it acts as a substraction:
20k heal -5k negated heal -5k damage = 10k heal. This is what we see ingame in these case.
Note: all the numbers are orientative for the examples.