I'm new to D&D 5e and monster weapons and attacks are confusing. I want to clear this up before I allow this, because it could be a huge mistake. 

If my PCs kill a monster and decide to take the monster's weapon, do they get everything that goes along with it from the monster action section? So as an example, I have just killed a Redcap and I take his sickle — does that mean I get the "wicked sickle" action which is a +6 to hit, reach 5ft for 2d4+6 slashing damage, or is it just a sickle? 

Really it is the modifiers that worry me. Some of these weapons have some crazy to hit bonuses and I kinda need to know if that is okay to let them have that or not.