[InspiredIdiot]

If the attacker's ultimate goal is ideological but their proximate goal is chaos and they think they have a large enough group of similarly motivated people the purpose could be quite simply to show that it can be done. Do it, don't get caught, and wait for the copycats. It doesn't even matter if their ultimate goals and the original attacker's align.

[LarryMullins]

Without a publicized motive, it might create a "stand alone complex" of copycats copying copycats without any unifying ideology between them. Each copying the other for their own personal reasons. I think school shootings work like this.

[robotnikman]

If someone wanted to cause as much chaos for whatever reason, that sounds like the way to do it as a single person or small group of people.