With three I am used for cleaning,
with four I am used to kill,
with five I can be pretty quick,
with all seven I am considered weak.
Hint:
The slices may not be in the same order
My answer is:
FRAGILE
With three I am used for cleaning,
RAG, a cleaning tool.
With four I am used to kill,
FRAG, a hand grenade.
With five I can be pretty quick,
AGILE, quick.
With all seven I am considered weak.
FRAGILE