The reason it is usually translated as "evil" is because the Greek word "πονηροῦ" is in adjective form. However, its usage in many of these verses is a bit unique, seeing as it is preceded by the article which would normally be expected of a noun.
Even in English, there are times when the article "the" may be used with a word that is not a noun. For example: "The more, the better!" Neither "more" nor "better" in this common saying would typically be considered a noun. However, when preceded by the definite article, it can become a noun--as noted even in the dictionary.
The same principle applies here to the Greek. It becomes a matter of interpretation on the part of the translators. In order to make sense in English as a noun, they might feel obligated to add a word that the Greek does not have, i.e. "the evil one." If the translators are sensitive about adding words, they may choose not to do this, and just leave the expression as is, with "evil"--of course, omitting "the", but Greek articles are not so precise as English articles, so they probably feel better able to justify this.
It is likely that the Greek author had both "the evil one" and "evil" in general in mind when penning the words; i.e. it may be double meaning. If one ponders for a moment, however, it seems more logical that we be delivered from a sentient being more than some abstract and inanimate thing.
Regarding the gender of the word "πονηροῦ", please consult the grammatical notations for this word as present in both Matthew 5:37 and Matthew 6:13, with screen captures of an interlinear Bible for these posted below.
If you look at those images carefully, you should see what I think I am seeing--identical spellings for "πονηροῦ", yet one being labeled as "neuter" and the other as "masculine". Neither does the preceding article "τοῦ" (which is not translated) show any difference.
It may be that this is just one of those forms in Greek that can be considered either masculine or neuter. I have taken fewer Greek courses than Hebrew, so I will have to defer to others on this question.