Most slayer monsters have a superior version of them for example Kurask have the King Kurask and Bloodvelds have the Insatiable Bloodveld and so on which have a chance of spawning while on your task.
i believe the superior versions should count as 2-3 task kills and give the same amount of xp drop as 2-3 kills, instead of just 1, it would be a good and fun addition to slayer as these superior monsters take longer to kill and are harder then the normal version of themselves.