He did.
He considers participants in the Labour Party to be "cultural marxists". He sees cultural figures of authority as traitors to his ideology, which he obviously considers superior. He classified these figures into Type A traitors, and Type B traitors. Type A was made up of political, cultural and religious leaders with a "culturally inclusive marxist orientation", whereas Type B traitors consisted of teachers, journalists, editors, and other similar positions of public responsibility of the same "multicultural marxist" orientation. He wrote a 1500 page manifest which documents the ideologies he supports, as well as a detailed, dated journal of the events leading up to the attack. It documents all stages of planning and execution and the most recently dated post was dated a few hours before it all happened. He had done a risk assessment with extensive reasoning. He lists all the chemicals used, the amounts, etc. He has also specified that recrutation into the labour party is the "head of the beast", which is why he struck where he did. It would be the most effective way to set "cultural marxists" back.
The document is titled "2083. A European Declaration of Independence". It is a call to arms. It is written as a guide on how to start the revolution, or something along those lines. It encourages anyone who is planning a similar "operation" (it is the word he uses to describe his actions) to pay heed to his risk assessment. With a 30% chance of getting off, sustaining a belief that there is hope of freedom afterwards will lead to insecurities, hesitation, confusion, and thus it would be wiser to assume neither possible outcome (death/captivity or freedom) as likely. But rather, use this fact as a motivator to execute the operation with maximum efficiency. In other words, he specifies that his intention was to kill as many people as he possibly could.
It also consists of borrowed texts from research, political "experts", political leaders, dictators of the past, and he has copied a declaration from the UNA-bomber and edited it to fit his ideology. It's quite extensive and he had very specific reasons. He wants to share it with the world tomorrow, and has expressed so to his defense attorney who relayed it to the media. As a matter of fact, from his risk assesment, he reasons in his manifest that his only way out would be through convincing the world.