Google has always maintained that websites spamming and stuffing the Meta keywords need not worry at all. It never leads to a penalty as Google simply chooses to ignore it all. Thus, a website can use as many Meta keyword tags in relation to the real keywords and it will not cause any kind of penalty.
A Twitter user wanted to be sure that Google is not going to penalize for having a Meta Keywords tag on the page.
Gary Illyes from Google answered his query saying that there is nothing to worry about Meta keywords tag and it will not lead to a penalty. He added that it does not have much of a use otherwise.
So, you are free to use the Meta keywords tag but you also need to remember that there are other search engines like Bing that may or may not use them.
See the forum discussion at Twitter.