To end up protecting the peace 〜ことになる

Grammar Explanation: To end up being with 〜ことになる

The trio Colors take a drawing of the panda-pattern monster cat to Officer Saitou to see if he knows its whereabouts so they can capture it to protect the peace in their district. Saitou is initially unwilling to help the young girls and admonishes them for wasting his time. But when Yui starts crying, Saitou relents with information. He says it’s a thieving cat. Lately, there’s been an increase in stories about a panda-pattern cat taking things.

斎藤(さいとう):: 「ま　だから捕(つか)まえようもんなら　この街(まち)の平和(へいわ)は守(まも)られることになるかもな」; “Well, that being the case, if you were to capture it, I suppose it'll end up having protected the peace of this district.”; Literal: “Well, so if you were to catch it, maybe this town's peace would become the situation of being protected.”
結衣(ゆい):: 「泥棒猫(どろぼうねこ)っ」; “Theiving cat!”

Key Points

1守られることになる describes the resulting state

If they catch the cat, the district’s peace will end up being protected.
The verb is receptive: the peace is what receives protection.

2捕まえようもんなら means if you were to catch it

The phrase sets up a hypothetical achievement.
Saitou is not promising it will happen; he is imagining the result.

3かもな keeps the claim tentative

かも means “maybe”, and な adds casual reflection.