So, Hiromu was raped by his dad. It was raining when his dad tied him in their stable and let the horse rape his son why he put his ding dong on his son's mouth– basically having threesome with the horse. So when his younger brother told him that their dad lover him, Hiromu snapped. His repressed sadness and rage made him vent it out to his younger brother that's why we can see him 'raping' his younger brother after saying 'what do you know?' when younger brother said that their dad loves him. Hiromu 'raped' his younger brother to show what their dad's love is but younger brother actually nows what happened and said 'I'm not raping you, I'm not Dad." And then asked what he wanted. Note that this is happening as it rained, thus the title. Although, we did not get to see what Hiromu's wish is after younger brother asked what he wanted, we can deduce that he wants to 'die' so he can be freed...that's what the last panels are.
For short, its fucked up.
Can't escape from getting his clothes rip, can't escape from getting locked, can't escape from the SSS-men