To what extent is my understanding correct regarding the hadith: "Whoever assists in the killing of a Muslim, even with half a word, will come on the Day of Judgment with 'despaired of Allah's mercy' written between his eyes," that Allah's mercy will not descend upon whoever helps an infidel kill a Muslim?

The aforementioned hadith (definition) is very weak and fabricated, and a number of imams have ruled it to be weak or fabricated. Despite the weakness of the hadith, assisting in the unlawful killing of a Muslim is considered one of the major sins. This assistance may amount to disbelief in some circumstances, such as supporting polytheists and aiding them against Muslims. One must not be lenient in attributing hadiths to the Prophet, peace and blessings be upon him, and whoever narrates a hadith knowing or suspecting it to be a lie is a liar. If a person cannot ascertain the authenticity of a hadith, they should ask the specialized people of knowledge.