Why don't you try this code? I think it was when you put M.HP = M.HP - damage

proc
doattack(mob/player/M in oview(5))
var/dmg = 5 - M.Def
while(M.HP)
if(M.HP == 0 )
M.Deathcheck()
break
flick("attack",usr)
sleep(2)
s_damage(M,dmg,"FFFFFF")
flick("gethit",M)
view(4) << "dekuhhit.wav"
M.HP -=dmg//This is the problem here
M.Deathcheck()
sleep(20)
var/mob/M
doattack(M)

s_damage(M,dmg,"FFFFFF")

If you're using an HTML colour code, you need the "#" symbol.

s_damage supports either literal colours, like:

"yellow"

or HTML colour codes, like:

"#99FFCC"


Notice the same is true for real HTML:
<font color="yellow"><FONT COLOR="YELLOW"></font>
<font color="#99FFCC"><FONT COLOR="#99FFCC"></font>

here is a much better way to put it





mob/enemy/thief

HP = 10
Def = 2
icon = 'thief.dmi'
dir = EAST
density = 1
exp = 20
proc
doattack()//changed this
for(var/mob/M in oview(5))//this
var/dmg = 5 - M.Def
flick("attack",src)
sleep(2)
s_damage(M,dmg,"#FFFFFF")
flick("gethit",M)
view(4) << "dekuhhit.wav"
M.HP -= dmg//this
M.Deathcheck()//why did u have 2 death checks I removed the top one but there was an error in it, in your death check make sure it is usr.hp <= 0 up there it was M.hp == 0 needs to be <=
sleep(20)
New()//this
doattack()//this


Entered this code but still the mob wont fight back. Thanks spuzzum for clearing up the s_damage problem.