Well, as Whitehall 1212 (who seems to have far more experience of serious criminal investigations than anyone here, especially suspicious hospital deaths) has said, that is not necessarily the case!
Being arrested, even on very serious charges, is not always necessarily very strong evidence of guilt. It merely means that the police have enough evidence/suspicion to arrest you. Indeed, it is not always a bad thing for the arrestee as certain protective statutes (such as PACE, bail provisions, etc) become immediately relevant. A now retired cop who was old enough the remember the situation pre-PACE was very adamant in telling me that if the police ever tried to call you in for a private chat about a serious crime then make sure that arrest you first. It also protects their investigation - wasn't there a recent case of a murder prosecution falling apart because the cop in charged spoke to the suspect before he should have done or while he wasn't arrested or something?