its also heavily implied in the horus heresy books that leman russ killed the two lost primarchs.
as far as the ranking go of deadliest primarch, you shouldn't take their kill counts against other primarchs because that is just dumb, take their actions from the fluff into account.
Heres the list of deadliest primarchs in a 1v1 fight
1. Angron- no contest, his sole purpose was to murder, nobody does it better
2. Leman russ- always on the front line leading the hunt, an absolute monster in combat
3. Fulgrim- perfection in combat, needed to be in combat more to truly shine though
4. Konrad Curze- use of terror and blender hands made him a beast, as well as front line action
5. Horus- even before chaos he was a force to be reckoned with, years of experience to help
6. The lion- devistating with a sword and a wrecking ball in close combat, shame he's a traitor
7. Sanguinius- not to bad in combat, but mediocre when combat to the top 3 on this list
8. Magnus- even with tzeetch behind him, couldn't beat leman, an ultra wizard to be sure though
9. Mortarian- hard to kill but he is slow and his skills can only take him so far
10. Vulkan- unkillable, but it doesn't help when you are a nice guy, sick wepons though
all other primarchs would be gods in combat against any opponent, but guys like perterabo, rogal dorn or alpharius were tacticians and used their mind to win battle and thats what made them so deadly, its up to you whether you think thats better or worse