Why is Rumpelstiltskin a bad guy again?

Short answer: Power.

The whole reason Rumple/Gold became what he did was because he wanted to be in control- powerful and fearless, not like the coward he once was.

However, in acquiring that power he also bound himself to the dagger, which means anyone with that dagger controls him- as long as they hold the dagger, Gold is forced to obey their command, as we have seen before.

He gave Belle a fake version of the dagger when he proposed so that he could be free to kill Zelena, who is indirectly responsible for Neal's death (his son) since it was Belle's wish that he not kill Zelena.

He tells Neal (his grave) about gaining love with Belle, but it has started with a lie since she believes the dagger is in her possession. Mr. Gold contends that the dagger was needed to avenge Neal's death, but now he must find the strength to return it to Belle.

He did initially return the real dagger to Belle, but took back shortly afterwards. After he last experience under the control of Zelena, which as noted above resulted in the death of his son, Gold wanted nothing more than to be completely free of the dagger but still retain his powers.

The apprentice (of the magic wizard's hat) believes Mr. Gold will never accumulate enough magic to reach his goal, which is to be free of the dagger's control and retain his powers.
...
The Snow Queen reveals the dagger was once in her possession and thus she knows its secrets. In exchange for knowledge of the last thing he needs to be free of the weapon's will, Ingrid wants the ribbons she and her sisters gave him in a past deal. After she whispers the secret to him, Mr. Gold replies that he'll gladly do it.

. . .

So basically the reason why he's still keeping secrets from Belle, giving her the fake dagger and retaining the real dagger himself is because he wants to be completely free of it- as long as the dagger exists and he is bound to it, there is always the chance someone could come into possession of it and control him like Zelena did. But he still wants to retain his powers.

**Gold wasn't really evil at all last season aside betraying Ingrid, and almost killing Hook. In respect? I thought it was rather poetic that the man whom fled with his wife 300 years ago, (and although the soldiers wanting to take Baelfire was the reason he chose to become the dark one could have been avoided Had Milah been there with her own perspective on what to do. So in that sense you could blame Hook for him becoming the Dark One to begin with) would be the one that would have to die to free him of the dagger's spell.

*I keep seeing Rumbellers not understand why Rumple wanted to keep his power and yet not ever be controlled again. Why can't he have just left for new york with her and started a new life? Well think about it. He needs Magic just to walk. And spin gold. Spinning gold keeps his sanity and Walking is a given. Keeping his powers in the real world is a must just so he can financially take care of her in an otherwise expensive world.

Rumplestiltskin didn't do a single thing out of line last season whatsoever as the hat itself only traps people for thier power doesn't kill them. He just wanted to be free. Hence to deem him evil for this is silly after everything he has already been through for this town. He DIED for the town to save them from his father. He got no thankyou. The towns people never bothered to comfort or rescue him. They used him for information and then left him to be kidnapped afterwards. He felt betrayed by the entire town already and they keep coming to him for his power. To use him and then ignore him again. If anything every single action he's done is entirely Just and understandable and not evil. Killing Hook was supposed to be the sacrifice for freeing him but thankfully it turned out to be a blessing in his case.

In conclusion this whole show has been a one stop train to never ending Screw over Rumple because he conducted a curse to happen to move everyone here so he could find his kid.**