Rick's group isn't the one that started it, the Saviors were. That group that Abe, Daryl, and Sasha ran into weren't just some group operating on their own. They were patrolling for Negan. They even stated that they usually just kill one person right off the bat when they encounter a new group so they understand the situation. Meaning, they would've killed someone regardless, even if Rick's group hadn't killed any of them. That's how the Saviors operate. They clearly don't just take what they need, they take what
they want. As Daryl said, "It's not about getting by [with the Saviors]. It's about getting everything."
Negan even stated that if you're working for points (the run of the mill Saviors who aren't working directly for Negan) you'll wish that you were dead. Does that really sound like a leader that cares about his people? Do you really think that Rick would be okay with his group being so miserable that they wish they were dead?
The attack on the outpost while they slept was a result of the Savior's M.O. If they were a peaceful group, like Oceanside, they would not have murdered them like that. Rick and his group went out of their way to make sure they didn't kill anyone at Oceanside. Do you really think the Saviors would have done the same? Was it smart to attack them when they new so little about them? Of course not. But as far as the tactics that Rick used, they were a response to how the Saviors operate. It was a premptive attack because they knew, with absolute certainty, that the Saviors would try to dominate them, exactly as they have.
If they didn't do that, yeah, maybe Glenn would still be alive, but someone else would be dead in his place and all of this still would have happened. That's how the Saviors work. They would have come after Rick's group regardless.