It's very hard to plain 1hit kill. You need a multiple of things happen:
1. HUGE speed bonus with you and your opponent running at each other
2. Head shot
3. Slight hold
4. Turn into swing
Even if you do all of this, there is a random factor to damage, and armor is so resistant to damage nowadays that still might not be enough.
Your 10PS is not bad at all.
On my 8 IF, 27 Strength, 55 armor character, 1 slash from you usually does at least 50-70% damage since you're good at timing your attacks.