we are talking about the time of the Russian Japanese war. Russia was already wanting to expand deeper into East Asia. They would have taken Korea if the Japanese did not take it.
Another question would be, What if Russia colonized Korea instead of Japan?
typical Japanese answer, did you know Korea had better relationship with Russia than other colonial powers?
Queen Min and her husband had a deal with Russia to modernize Korea with Russian just before she was assassinated by Japanese and King fled to Russian embassy to save himself. Thanks to Russian eye witness accounts, we have detailed records on how Queen Min was murdered by Japanese.
Russia would have made Korea as their ally to destroy Japan, and this scared Japanese.