Mohammad: Executioner via Amputation
I have previously mentioned some of Mohammad's questionable actions here: Mohammad: Sex with 9 year old -- where the account of his marrying a 6 year old, and consummating the marriage when she was 9 years old.
In Islam, Mohammad’s life is the best example of how to lead a virtuous life and how he lived is to be emulated by all faithful Muslims. (Surah Al-Ahzab 33:21).
It is blasphemous to even suggest that Mohammad’s moral example could be improved upon.
That being said, consider the following passage from the Sahih al-Bukhari (the most trusted Sunni collection of Hadiths which records the life of Mohammad):
The Prophet cut off the hands and feet of the men belonging to the tribe of 'Uraina and did not cauterise (their bleeding limbs) till they died.
Yet another example of moral exemplar Mohammad in action...
Argue something like, ‘in light of Mohammad’s killing the men of the Uraina tribe by letting them bleed to death after cutting off their hands and feet, the moral example of the life Mohammad lead could be improved upon for today’. The claim that is being made is narrow, focused, and does not require too much to defend.
Tactic: Argue for the most defensible claim that is fundamentally devastating to your opponent’s position
In bringing this up in an argument, I wouldn't worry so much about the fact that he was killing the men of the Uraina tribe. Stick to the fact that, even if these people had to die, Mohammad chose a particularly horrific way to kill them.
This argument, like the argument involving Mohammad having sex with a 9 year old, is most effective in a public setting. When Muslims say that they should emulate Mohammad's life, they do not mean that some of the things he did were 'forgivable' but that his actions typify the highest moral life. The situation ends with the Muslim either publicly defending killing all the men of a tribe by letting them bleed to death or publicly stating that Mohammad’s example needs some polishing to be applicable for today.