Should be killed with 〜べき

Grammar Explanation: Should be with 〜べきだ

Yui, Sacchan, and Kotoha are friends who’ve formed a group called Colors to protect their neighborhood. Yui is the group’s leader.

Yui hurries to the Colors clubhouse with news of a cat she saw. She draws it on paper, but Sacchan sees it as the sketch of a monster. Kotoha looks over the drawing and asks if they’re going to kill it.

結衣(ゆい):: 「こ　殺(ころ)すのはダメーッ」; “K-killing it is not okay!”
琴葉(ことは):: 「でも　こんな化(ば)け物(もの)ブッ殺(ころ)すべき」; “But a monster like this should be killed.”
さっちゃん:: 「そうだ　じゃあ捕獲(ほかく)ならいいかリーダー」; “Right. Then is capturing it okay, Leader?”
結衣(ゆい):: 「うんそれ！それにしよう！」; “Yeah, that! Let's do that!”

Key Points

1ブッ殺(ころ)すべき = should kill

べき attaches to the dictionary-form verb ブッ殺(ころ)す, so Kotoha is stating what she thinks the proper action is.
The subject is omitted. In English, “a monster like this should be killed” keeps the focus on the monster rather than naming who should do it.

2べき gives a judgment, not a hard rule

Kotoha is not quoting a law or requirement. She is giving her blunt opinion that killing the supposed monster is the right choice.
That fits べき’s nuance of ought to or should, stronger than casual advice but short of must.

3こんな化(ば)け物(もの) frames the target harshly

こんな means “this kind of” or “like this”, and it points at Yui’s drawing.
Calling it a 化(ば)け物(もの) makes Kotoha’s べき sound more justified from her point of view.

4ブッ殺(ころ)す is rougher than 殺(ころ)す

The prefix ブッ adds a forceful, violent feel, closer to “kill outright” than the plainer 殺(ころ)す.

5捕獲(ほかく)なら shifts the proposal

Sacchan changes the action from killing to capture with じゃあ捕獲(ほかく)なら, “then if it’s capture…”