I too don't believe the accused went there with the intention of murdering 3 people, I also don't believe he went there drugged (as there has been mention that this seems unlikely) or angry. Drunk perhaps, but could a drunk teen commit such brutal crimes?
I don't believe he went there angry, as the person who dropped him of would have sensed/known this and one would imagine have alerted police imediately. Some have suggested that maybe the police had him in their sights from early on, but I doubt that if the driver told them he had dropped him off their angry and he had cuts on his arms (as has been stated but only as rumour), they would simply let him go and wait for Dna evidence.
Many a case has been based solely on circumstantial evidence. What if the Dna came back inconclusive. If the police had reasonable belief that he was the perp, he would have been arrested immediately.
Some facts on arrest in Sa:
What procedure must be followed if a person is arrested without a warrant?
People apprehended without a warrant must be delivered to the nearest police station as soon as possible [Summary Offences Act 1953 s 78(1)].
However, a person arrested without a warrant who is suspected of committing an indictable offence or an offence punishable by imprisonment of two years of more, can be detained for as long as four hours to investigate the offence. The detention can be extended up to eight hours if authorised by a magistrate. During this time the person can be taken to places connected with suspected offences to assist police in their investigation [Summary Offences Act 1953 s 78].
http://www.lawhandbook.sa.gov.au/ch02s02s03s01.php
Reasonable cause required for arrest to be valid
Generally an arrest will be lawful if the officer who carries it out has reasonable cause to suspect an offence has occurred or will occur. The arresting officer may be mistaken as to the facts, and the person later released. Provided, however, the officer had an honest and reasonable belief in the relevant facts, then they have acted lawfully. For that reason no attempt should ever be made to resist arrest. Civil action can however be taken for damages or compensation if a wrongful arrest is made.
http://www.lawhandbook.sa.gov.au/ch02s02s03s02.php