This teacher is just an asshole but the relationship between teachers and students is complex and most of the issues are systemic. In the same way that a retail or food service worker may hate their customers because of how some customers act, it should not be surprising when teachers may come to hate 'students' in the abstract when the students are what cause them the most difficulty on a day to day basis. Now some teachers are able to introspect and understand that the students themselves are not the root cause of these problems.

People outside of education often think teachers have control over their schools and students. However that control is mostly a facade, teachers increasingly feel powerless in their own classrooms as society cuts more and more support positions forcing teachers to pick up the slack. Students with exceptional learning or behavioural needs are left in overcrowded classrooms and obviously this causes problems that are not the fault of these students.

Most teachers are only in the system because they care about the kids. I wouldn't look at the teachers subreddit and expect a real look at how these teachers think. Most (not this guy) are mainly venting and coping using dark humour as the system collapses around them, much like how outsiders are surprised at how medical workers use dark humour to cope.