That last exchange can be summed up as projection:
From this fantasy, Slant then worked up his rebuttal to an imagined enemy.You sound like you want IS to be following legit Islam so you have a reason to rant on about how Islam is bad.
This is sadly not uncommon on these exchanges.
Wishing for Muslims to be able to drown ISIS and similar propaganda is not bitching about Islam or Muslims.
This is often verbalized as "Islam needs reform from within". Which is not a particularly surprising way to think: every ideology is said to need reform from within.
Questioning why they need it, and towards what end is what should shape that debating angle.
Imagining that the "why" is because "is bad", is a radically pointless way to conduct an exchange.
That a group of people gather to say "X is bad" doesn't really mean much to anyone other than the people gathering. Big deal. The question that should follow is "what are they going to do about it". And they can do many different things. Among others: take military action, host a book club, praying, getting drunk... or propose ideological reform. This last one is what many non-Muslims would like to see, possibly because they've seen it work within their own religion. It surely is not enough to cut terrorism and propaganda, but it's a tool nonetheless.