Some druid help.

[Belzyk]

So I need a little help guys. My 3rd level druid is about to slowly begin turning evil. Can I acompish this and still stay a druid or do I need to mabye dip into warlock or wizard? My goal is to eventually just reveal mid fight his transformation into this evil being.

[Hruken]

There are no alignment restrictions for druids in 5e, so you are fine there. Heck, you could be an evil druids in older editions.

[nickl_2000]

A druid who straight out murders anyone who encroaches on the natural forest would still be a forest protector and extremely evil.

[RickAllison]

"I want to flood the world. The surface is a plague that harms the ocean that birthed all life."

Evil goal (killing all life on the surface), effectively omnicidal toward anything that the players will commonly interact with (consider freshwater creature sell-outs for bonus points), and fits perfectly well with someone deeply in touch with nature.

Or be an advocate for survival of the fittest. Actively incite natural disasters so that any civilizations that are unable to handle them will we wiped off the planet.

Just look on the Well-Intentioned Extremist page on TV Tropes, you will find plenty of suggestions.

[Yuroch Kern]

There is also the Blighter angle. It was introduced back in 3e, a corrupted druid that sought destruction for its own sake, rather than balancing it out. Usually druids that start favoring certain climates/terrain, like the ocean example above, as the one true ideal also can go all evil in regards to their twisted beliefs. A fanatical druid is pretty scary. The Iron Kingdoms have great examples of that.

[Asmotherion]

If you're asking if you can stay a druid as an evil character, there is no restriction to that.

Now, if you want your druid to multiclass for story reasons it's diferent. Sure, a couple of Warlock Levels is a mechanical benefit, but so is staying Druid all the way.

Lore wise, you can make a Druid who made a pact with an Evil Fay (for example) to protect nature, but slowly get to the conclusion that humanoids are the reason nature is under threat, and see all civilised humanoids as your enemies. Device a plan to end civilisation, and go through with it as a series of (secret) side quests. Be warned though that, if the party does not agree with your character, they may turn on you, and your character may become an NPC if found out.

[Beelzebubba]

You can do it just with your roleplaying choices.

How about: pursue your evil plot in down-time and in various little ways while going along with the rest of the party, and maybe start being a bit more cruel or self-centered or strident, and see where it goes. And who notices, or cares.