A 70-year-old man was beaten and the attacker died of a heart attack. Should the elderly bear criminal responsibility?

Judging from your complaint, I don't think it is appropriate for the Public Security Bureau to file a case for intentional injury. I suggest that you lodge a complaint with the higher authorities or the police station inspector unit on the grounds of unclear facts and insufficient evidence, and ask the public security organ to re-investigate. If you are not satisfied with the physical examination results, you may ask the higher authorities or designated authorities to re-examine. You should bear the civil liability for the accident in criminal law, but I don't know the specific case. I don't know if you have hidden any unfavorable factors, so I can only reply according to your description.