Game mechanics of the vampire: For me difficult to understand why first live leech is useless if oponent has monster with weaken

A note on the game mechanics of the vampire:
Just had a fight where the vampire hit in the first round and also Live Leech +1 was displayed.
But then he was killed by the next monster that has only 1 damage.
I had to watch the whole thing twice until I understood it:
The opponent had used a monster with woks to deduct one life point from each opposing monster.
However (and I like that), the game mechanic provides that a monster (like the vampire) that has only one life point does not receive a trigger, otherwise it would have died before the fight started.
Obviously, then, the Vampire will immediately deduct the gain from the +1 live Leech from the enemy Monster with Weaken.
For me, the whole thing is difficult to understand, because it leverages the Live Leech +1.
In my opinion "weaken" should only be used at the beginning and not during the fight.
Since the monsters are marked with weaken yes (to re-credit their life when the opposing monster weakening dies), you would be able to don't select monsters that have only 1 life at the start of the battle.
What do you think about that ?
I would also like to have a controversial discussion, if perhaps one or the other sees it differently.