Making a custom lifesteal sword

So I'm making a custom sword that a remake from Kings SMP and basacly (great spelling ik) while you have the sword when you kill someone you gain a heart so I made this version where we detect if you have the right sword and then if you do we now count the amount of kills and we transfer that to a /attribute commands. 
My problem is that i have another item that for a short amount of time change the attribute of max healt to 12 and then back to 20 but the way i codded the current system for the sword fully bypass the abbility from an enemy