If you believe that you are a danger to yourself or someone else then go to a hospital immediately. The hospital can keep you safe. When you go to the hospital tell them what you wrote in this letter which is that you have a strong urge to poison someone. You should also inform them that you have harmed someone in the past and that you’re worried that you may do it again. They will likely hospitalize you and you can get the help you need immediately.
My main concern is that you seem unable to stop yourself from harming people. You have succumbed to your impulses in the past and that is concerning. You may need to be hospitalized at this time but you also should consider entering an intensive treatment program. Perhaps it is the psychosis that is driving you to want to hurt yourself or someone else.
Perhaps there are other explanations for why you want to harm yourself or another individual. As you mentioned you feel a sense of pleasure when you think of hurting someone. That sense of pleasure needs to be addressed and extinguished. That may be accomplished with a partial hospitalization program, a competent therapist or a team of therapists. The desire you feel to harm others or yourself is not something that should be ignored. It is not normal or healthy and it may be dangerous.
As I mentioned above, if you believe that you may harm others or yourself then go to the hospital. If there is no immediate danger then the hospital may not be helpful but treatment for these issues is strongly recommended. You should contact a therapist or an intensive treatment program as soon as possible and enter treatment. A therapy program could help you address your issues. Perhaps you could learn to redirect your unhealthy impulses toward a healthier outlet.
I am encouraged that you recognize that there is something wrong. I hope this recognition means that you are willing to attend treatment. You have already harmed someone and were punished, albeit not very harshly. At this point in time you recognize that those destructive thoughts have returned and you feel the urge to repeat your actions. Now is the time to do what is necessary to prevent a tragedy. At this juncture, your thoughts are only thoughts. Thoughts cannot harm another person but they may eventually lead you to engage in a serious or deadly crime. It is your responsibility to seek help for this problem. If left unchecked there is a chance you could seriously harm or kill another person and may be imprisoned for the rest of your life. You may even be given the death penalty. For the sake of yourself and the lives of innocent individuals please seek help immediately. Here is a website that may assist you in finding a therapist in your community. I thank you for your question and hope you will get the help that you need.