even though you're increasing _dammage value you're not 'setting' it
#bleeding
_man setdammage _dammage~ 1
_dammage = _dammage + 0.1
? _dammage = 1 : goto "end"
? _dammage < 1 : goto "bleeding"
And may I suggest a little less fast loop. With 1 second delay the _man will die under 10 seconds when adding 0.1 of damage to him.
It is possible to cacreate blood to the units position. Search this site for OFPEC Blood addon and learn from it.