It looks to me as though you tried to decal over a matte paint finish. Decals only really work properly when applied over a gloss finish. My personal finishing method is this:

- Allow the paint coat to cure at least for a day and then gloss coat with Future.

- Apply the decals and allow them to dry at least overnight.

- Overcoat the decals with another coat of Future and allow this to dry for at least a day.

- Apply any washes you're planning on using to the gloss surface. Washes flow better over gloss and    you'll  avoid "high tide" marks. Let it dry for at least a day.

- Apply as many light coats of matte/dull varnish (my favourite is Testor's Model Master) until you achieve the surface finish you're looking for. Dry for a day.

- Apply weathering powders/pastels/dry brushing over the matte surface.

You should be able to remove the adhesive staining on your model with some warm water and a soft rag. Sadly, the decal itself is probably a write-off.