My parents did not look after me well in the past. Must I still be devoted to them now that they are old?

Your parents' right to be treated well remains great, even if they were imperfect in raising you. Being good to them is obedience to Allah and a sign of your own nobility, not an endorsement that everything that happened before was right. This does not mean you let yourself be wronged; you can care for them while setting healthy boundaries to protect yourself. Start with what you can manage, and tend to your old wounds, perhaps with a counsellor's help, so that your devotion is not mixed with resentment. Pray for an open heart. Many find that forgiving frees themselves first. See counselling.