SIDEWAYS, is a character of mystery!  Depending on where you consult, you'll even get a different perspective on the nature of his personality!  Listed below is the character profile from Hasbro's Official Transformers web page. 

SIDEWAYS is a conscientious, ninja-like warrior who races to battle. Disguised as a sports bike he exceeds speeds of 250mph. Mysterious and silent, he reveals little about himself except a fierce drive to confront the enemy and quickly return to CYBERTRON. On Earth, he undergoes a personality change that gives him both a good and a bad side based on Mini-con influence. This split makes him highly unpredictable. It's hard to predict which half of his split personality will win in the end - the good side or the evil side!

Strength:  4.0
Intelligence:  6.0
Speed:  6.0
Indurance:  5.0
Rank:  6.0
Courage:  8.0
Firepower:  6.0
Skill:  8.0

According to the shows, however, the Mini-cons Rook and Crosswise are not the source determiner of his acting for either Autobots or Decepticons.  It seems that for Sideways his allegiance is more a personal choice based on his personal agenda!

Among the few redeeming qualities of the Transformers: Armada cartoon series' first half, Sideways is among the most interesting and intriguing characters written.  Shrouded in mystery he fights alongside Autobots on some occasions and with the Decepticons in others.  Meanwhile, his true allegiance is unknown. Or is it?!?!

In actuality, Sideways is loyal to neither Autobots or Decepticons.  His true loyalty, lies with (oh, we were just SO excited to learn this!!!!) UNICRON.  His purpose in the Armada battles is to keep an eye on both sides, to keep the battles ~even~ and thereby perpetuate the war thereby weakening both sides as Autobots and Decepticons destroy one another, until UNICRON's arrival!