Just make a trigger that triggers upon kill. When it is triggered, it checks if the killing unit has the ability (use integer comparison, level of ability for killing unit not equals to 0). Then set the strength of the killing unit to current str+(Some Arthimetic). I think it is possible, otherwise create tomes and give it to the killer but that sucks :S
For the bonus, just use arthimetic and put (your base str bonus for level 1)x level of skill for killing unit.
PS: I felt like giving +reps so I gave you one