First things first: you did the right thing in finding an editor for your manuscript, as it increased your chances of a publisher paying attention - you'd be surprised how many authors do not take such a step and it usually means they don't ever get published.
Now to the agent thing. There are a few little issues here ...
1. In my experience the contracts offered to agented authors are substantially different to those offered to unagented authors at most (not all) publishing companies. You could ask an intellectual property lawyer to look over your contract if you're concerned, although that may end up costing more than an agent's commission. You may also feel that you're really happy to manage the business side of writing as well as the creative - some authors love doing it. Those who don't usually have agents.
2. If your novel is fantastic, maybe more than one publisher would be interested in it. Just because this particular publisher is keen, are they necessarily the right publisher for your book? The right publisher is not always the first publisher who says, 'I love it!' or the publisher who offers you the most money. If you've written historical romance, say, and the editor you've been dealing with personally loves it enough to persuade everyone else at the Acquisitions meeting to publish it but the company has never done historical romance before - they specialise in thrillers and crime novels - are they really going to do the right thing by you? What happens if the editor leaves and your champion is gone? (This does happen and can have a huge effect on some authors.) There may be another publisher who is better for you.
3. For me, at least, taking on a new client means managing their career, not just one book. If you're a first-time novelist, it's crucial to get things right - or as right as possible - for that first book or your career can be over before it begins. (If you don't intend to write more than one book, though, you can ignore this bit ...) You may be able to get advice from someone you trust and that person can play the same sort of role I do for my authors - which is, truthfully, more about giving advice on career/writing/life than negotiating contracts - so you wouldn't need an agent. It is important, though, to have someone to talk to about the business of writing and things that come up in the creative process, regardless of whether that person is an agent or not.
Fundamentally, it can't hurt you to submit to agents at this stage - and in your cover letter you should mention that your manuscript is being seriously considered at a publishing company (name the company). If an agent wants to take you on then you can still say 'no' if you ultimately decide you don't need one. The one thing I will stress is that you need to put aside the excitement of thinking you may get published and look down the long road of your potential writing career. Think about what you need to do to have a career - what will be required to keep you happily productive for years to come. If handling the business side of things seems like it would be too much work and would distract you from your writing, you need an agent. If not, you don't.