I've been going through Comics.Comixology.com and I'm just not sure where to start the series. Can anyone point me in the right direction?

You should start with My First Maniac and Me Without You, they tell Cassie and Vlad's origins. Then you should follow the new series that just started not too long ago, issue 7 is coming out this week. There were 13 one-shots and a 32 issue series published by Devil's Due that you can go back and read if you want to get caught up on everything.

Its weird because euthanized has the earliest date. I guess the rereleasing screwed up.the dates.etc?

ok, so basically resurrecting a dead thread here, but I couldn't let this go, Hack/slash started out at devils due publishing with a few oneshots starting with hack/slash euthanized the chronology is as follows

Hack/Slash: Euthanized, Hack/Slash: Girls Gone Dead, Hack/Slash: Comic Book Carnage, Hack/Slash: The Final Revenge of Evil Ernie, Hack/Slash: Land of Lost Toys, Hack/Slash: Slice Hard, Hack/Slash Vs. Chucky, then theres the first ongoing series from devils due, then Hack/Slash officially moved to Image and you've got My first maniac (which is a prequel) and then the new ongoing series which as of this writing is on issue 13, honestly if continuity is an issue it'd be easier to just grab the omnibus' from an online retailer or something, it has all of them in chronological order