ISIS is using terror tactics to control its members, just like it uses them on the general public. This group will eventually self-destruct. No organization that feeds on hate and fear can survive for long in a civilized society. The barbarity of this group will eventually be contained and suppressed.
No, ISIS is not necessarily turning on itself when it executes some of its own members. ISIS has always used extreme and very violent tactics to achieve its ends, so assassinations of its own members is within the realm of what should be expected. There spats to still be plenty of recruitment to ISIS and violence around the workforce attributed to them, regardless of their tendency to occasionally brutalize their own members.
Many organizations with such strick rules do things like this to close in numbers in the rank and to make sure that everyone is following the rules. This is likely a tactic that is designed to help make ISIS stronger. If it does get worse and the rules get stricter it might self-destruct, but otherwise, no.
I think these tactics are the tactics that an evil and violent extremist group would use to control its people. By showing other ISIS members that they can be killed for disobeying orders or for not living up to expectations, it forces the other members of the group to fall in line and to avoid questioning their orders or their group's mission.