Why did Douglass believe he needed to use forceful language?

I believe that you are asking about something that Douglass says in his Fourth of July speech at Rochester, New York.  In that speech, he says "I will use the severest language I can command," and I think that is what you are talking about.

He believes that he needs to use this kind of language because of how evil slavery is.  He does not think you can sugarcoat it at all.  He thinks that he must speak out and say that slavery is opposed to what America is about and that it is opposed to what Christianity is about.

Because he thinks it is so evil, he feels he must denounce it as strongly as possible.