If youre intent on ruining your pure () then I'd first do Priest In Peril, Nature Spirit, Recruitment Drive, and Fremmenik Trials. Then youd be 20 Defense and you can try out Initiate Pure PKing / Staking. From there, you can become a rune/void pure by doing dragon slayer and claiming 20k of the MM reward. Then go full berserk and finish off the defense quests
do nightmarezone quests and afk in nightmarezone. I got 90+ stats in a few days there. Downside it costs alot in prayer pots but you can always do the absorbtion method. Nice account so far though :]
Personally (speaking from experience) I'd start a new account if you want to play a zerker. Once you get those defense levels you can't go back. It's up to you though. If it were me, I'd max out range and Mage as well as 75 attack before making a decision of going zerk. It's just a game though and you can always make another account. Just my 2 cents
P.S. I agree with them with getting prayer up and training via NMZ.
You can do lots of things with an account like that, you could just make str and keep the def/attack the same, or you could get 20 defense and make an initiate pure, or you could make a zerker. Your last option would be a 70 def account for pking with barrows. All about personal preference though