Am I wrong?, ...I just need your opinion Options

[Machiavelli]

I know that topics like these are not favoured in forums but I just came back from a nice SR-Session that went out...let me say...unpleasant. Basicly it was a run that forced us to defend a shelter/hospital for SIN-less in Puyallup (the bear-shaman who led this hospital wanted us to use non lethal weapons) I think you know this Crypta-thing in the Puyallup Barrens? The "enemy" sent us some mercs in two vans, heavily armored, one mage and 2 elementals. We had a little discussion with them that resulted in "you get 5 minutes to pack your things and leave" which left me unsatisfied. So i called their boss again and asked him to contact his Johnson, because we would like to make him an counteroffer and a fight would lead to wounded or even killed people on both sides (and no, i haven´t been the politest guy on earth). He said "Ok, I will contact Mr.J, give us 5 minutes". Suddenly his men went onto us, searching for cover and coming closer. Nothing made me thing that this was anything else than an attack, so I unleashed a little mana-blast that incapacitated one of the mercs. Unfortunately they had a sharpshooter who killed 2 of our men quite instantly. This led to a little discussion afterwards that I should have had to wait until they opened the fire. But to be honest, armed people approaching in cover into my direction don´t make me ask for the first shot. So would you have handled it the same way or am I fact totally wrong? Would you have depended/trusted that they use non-lethal ammunition?