People also couldn't carry out the kind of horrific acts being inflicted upon Palestinians in Gaza or the West Bank without seeing them as less than human too. Supporters of the Israeli government and IDF have explicitly stated that they view Palestinians as sub-human. Some extend this out to Arab people in general, or even to Muslims in general. People have even encountered this view freely expressed by supporters in the UK. That is how normalised and accepted this viewpoint is considered to be in some groups.
The publication of cartoons, social media posts, etc using dehumanising language and imagery contributes to spreading and reinforcing the acceptability and normality of dehumanising people. The imagery may depict one specific example of a disliked or hated person, but the message conveyed by such imagery is clear, and intended to be generalised - just as a cartoon depicting a prominent and unpopular black person/gay person/Jewish person/etc as an animal like that would be condemned for its prejudice and contribution to dehumanising a group, we should also be able and willing to say that it is wrong to use such tactics against Palestinians, Arabs and Muslims.
Things like "cockroach", "rat", etc are dehumanising insults people have a long history of using against Palestinians, Arabs and Muslims. Their use in media supporting the IDF's actions is intentional and very telling.