In investigating the claim that Nigeria is killing record number of Christians, while the fact that in Nigeria a large amount of Christians got killed is true, the intention of killing these people was not intentionally towards Christians. Therefore, the claim is true, but misleading in some perspectives of the fact.
In the article presented by the fact-check is a well known magazine "BBC", the reporters explained very thoughtful every process of their news, explicitly explaining that there are several civil wars happening throughout Nigeria. Also, the Nigerian government has explained that the terrorism happening is not towards Christians, the terrorism happening is against everyone.
Nevertheless, the president of the United States "Trump" has been threatening the Nigerian government that if they do not stop killing christians, the United States Army would take charge of the situation getting involved. This claim by the president helps reinforce the idea that in Nigeria are specifically targeting Christians.
I have found multiple articles providing the same information, not more, and not less. All of the articles back up the main article presented by the fact-check. One that I found really useful to understand the problem was from "International Crisis Group", in which the reporters explain in detail every step by step of the White House making the decision to take action in Nigeria or not.
In conclusion, this fact-check is true but misleading. Even though, there are some information backing up that Nigeria killed a record amount of Christians, the several civil wars going on in that country point out that killing is not targeted towards Christians, indeed is targeted towards everyone near the zone at this time.