Why are some of his films are so horrible?

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He didn't direct 28 Weeks Later. Unless you made an error and in fact are talking about 28 Days Later which he did direct.

Because he's just not that good. He's a really undisciplined filmmaker that applies flashy technique and audio-visual gimmicks when none are needed. His movies are more often stylistic indulgences that smell of an older filmmaker trying to remain on the cutting edge and failing.

If he has a strong screenplay and a good cast then his movies survive his horrible directorial decisions; for instance Mr. Roe's Virgins, Trainspotting, 28 Days Later and Slumdog Millionaire. But a lot of his films are either mediocre (Shallow Grave, Millions, Sunshine) or actually terrible (A Life Less Ordinary, The Beach, Strumpet, Vacuuming Completely Nude in Paradise, 127 Hours, Trance, Babylon).