Is it sound to think obsessively about harming others, and to fear causing their death, and consequently entering Hellfire because of that, and how can one be rid of it, especially with the 'Umrah journey approaching?

This line of thinking is a satanic whisper (waswas) aiming to disrupt your life, so pay no attention to it, repel it, and go on with your life normally. Go for your Umrah, and exert your utmost effort in helping people and being good to them, for this is one of the greatest sources of goodness. Do not let Satan stand between you and that with his delusions. We advise you to consult a trustworthy psychiatrist.