I didn't want to tell it to you before because I thought it might prejudice your decision unfairly. If you were a good person who never killed or lied, but you had to do one or the other, which would be worse?

Notice how Harry doesn't want to "prejudice" Draco with a favorable truth?

I understand the impulse to shade the facts against yourself when being judged by others, but it isn't really fair to you or the person judging you.

I don't... don't want this anymore, I don't want to be manipulating you. I've hurt you too much already.

Because Harry's conscience pains him, he'll ask the 12 year old Draco to make a hugely life altering decision when he's in shock and hopeless about the future, and give him half an hour to do so, then hide that decision from him so that he can neither recant nor even know that he ever had such a choice, or made such a decision.

Time enough for such a decision after Draco had been reunited with his mother for a while. That talk, and that decision, could have waited til the start of the next school year, and Draco could have been given more time to make the decision.