Is one who calls for the death of Christians considered an infidel, knowing that he did not mean Christ, son of Mary?

There is no harm in supplicating against disbelievers in general. The qualification the questioner mentioned regarding his intention does not need to be stated due to its clarity. The questioner's doubt about disbelief is unfounded, for those against whom he supplicates are the enemies of the Christ, not his followers. Therefore, his supplication does not include the true followers of the Christ who believed in him and followed him, and who were upon that before the advent of the Prophet Muhammad, peace and blessings be upon him, let alone the Christ himself being included in this supplication.