I propose that instead of straight percent of targets health which is either always good or always bad the passive gets reworked to something like this:
Does a minimum of 2% of your targets current health or 10 damage.
Does a maximum of 10% of your targets current health.
Damage dealt is based on a ratio of your max health to your targets max health.
The ratio gets cut off at 0.
Ratio = ((targetMaxHP - yourMaxHP)/targetMaxHP)
Example TargetHP = 4000 yourHP = 2000
Ratio = ((4000-2000)/4000) = (2000/4000) = .5
range is 10% - 2% = 8%
8% * .5 = 4%
2% minimum plus 4% means you deal 6% current health
Example 2 TargetHP = 4000 yourHP = 3000
Ratio = ((4000-3000)/4000) = (1000/4000) = .25
range is 10% - 2% = 8%
8% * .25 = 2%
2% minimum plus 2% means you deal 4% current health
Example 3 TargetHP = 2000 yourHP = 4000
Ratio = ((2000-4000)/2000) = (-2000/2000) = -1 capped at 0
range is 10% - 2% = 8%
8% * 0= 0%
2% minimum plus 0% means you deal 2% current health
This makes this item efficient at killing tankier people, and terrible at killing anyone squishier than you.