Can I decline the previous company?
Of course you can.
Would that be okay? Or it's a really big dick move?
From the previous company's point of view, yes - it's probably a "big dick move".
But do you care? Even though you accepted their offer a while ago, you still have talked to other companies. Why the remorse now?
I haven't signed any documents or anything, I just said I accept the
offer when they called me to come to their office. I was afraid to
tell them that I might seek to work for other companies.
That was a mistake. But it's a mistake in the past. You should deal with the now.
This isn't a terribly unusual situation. Companies make "future offers" verbally to students all the time, but usually know that students are young, and things change rapidly for them.
The company will be disappointed, but won't be shocked.
If they had given you money, or had you sign a contract, the expectations might be different. But until you actually start working, prospective employers always worry that the candidate will actually show up.
You may be burning bridges with the previous company, and the previous hiring manager, but these things happen.