But he was on a high with his drug intake. Remember he was squeamish by nature, hence his practising with strangling rats. He had a time window from establishing his alibi by watching Man Alive to walking or cycling home under the cover of darkness. When the drugs began to wear off he was already back at Goldhanger ready for the next stage of his plan.
Even with gloves strangling rats is extremely risky, he would have to trap them first as catching them in the wild is next to impossible.
I stood on one once and it fled, but cornered they can attack.
Killing rats in no way would prepare him for killing humans anyway.