The point is that they obeyed Peter

TG:No Peter did not decide that on his own, HE knew what the Scripture said about another taking the place of the traitor. Don't you READ?
Pay attention:

Ac 1:20For it is written in the book of Psalms, Let his habitation be desolate, and let no man dwell therein: and his bishoprick let another take.21 Wherefore of these men which ....

And the Psalm is:
Ps 109:6 Set thou a wicked man over him: and let Satan stand at his right hand.
7 When he shall be judged, let him be condemned: and let his prayer become sin.
8 Let his days be few; and let another take his office.....

He also drew lots, which is also prescribed in Scripture. The point is not that he used Scripture. The point is that they applied Scripture according as he prescribed as the appointed authority over the Church. He, Peter, the Prince of the Apostles, commanded the Church how to appoint the replacement for Judas. He didn't ask them what should be done next. He explained what they would do next.

You really should read the whole Psalm it is prophetic in more ways than one.

You really ought to read what I say instead of what you want me to say. 

Sincerely,

De Maria