To me Steven is worse than Leo, as he obviously can't control himself. At the end of the day, Leo was trafficking drugs, not consuming them.

And Steven look somehow... Don't know how to put it correctly... Unclean and filthy in the most depressing addict's way. As he haven't take a shower in years.

I guess you can say that Leo was always violent and mean though, while Steven in his first appearance looked a little more like Bobby in the original show. It's true that we never saw what Leo was like when and before he and Shelly got married. Also I don't think we ever saw Shelly doing any kind of drug. I guess they can do some kind of mix between Laura/Shelly and Bobby/Leo.

This definitely screamed of Leo/Shelley 2.0 as spyros put it.

It is entirely possible that Becky didn't actually "do" anything that he has to be mad about.  He's just a junkie who's probably paranoid and crazed and likely a classic abuser just like Leo was.

I can't help but think the red coffee cup he flung out of the window may offer a clue. He sure seemed to believe she was up to something, "I know what you did!" And his rage over money, "You barely even make minimum wage!!" Could he have discovered that Becky might not be as innocent as she looks? Maybe she's found a way to supplement their income.. At One Eyed Jack's, maybe?

Quite possibly. Or she could be working for the latest Renault brother. Shelly certainly can't keep funding their "practices."

I thought that he expected drugs somewhere in the house and suspected she used them.  Maybe that is his paranoia, e.g., he consumed them but doesn't recall.  Maybe it was money missing.  Either could be in the cup.  Or maybe the cup is just something of value to her that he throws out as he was rummaging through the house.  He would bring up money in anger since he wants a fix and neither can't afford to replace the drugs.

I suspect it's a clue, based on the way it landed and how it stands out, but not sure what.  I would expect Carl to approach it sometime soon, maybe after Steven leaves to get his fix, which could either involve him talking to Becky or somehow otherwise adding to the story.

Throwing out the red cup is end of their mutual desire for one another, the 'gold' enjoyment(cocaine, promiscuity, etc.) took over at the expense of their private relations.  My first thought was that Becky, because of her habit, cheated on him for someone with more money and drugs, maybe Red(who smirked as he said, he 'really liked the town', after telling Richard he is already in with his lady friend, sending Richard later into that crying to rage scene in the truck) or Jean Michel.  I am saying this because Becky seemed to place excessive importance on enjoyment(gold) rather than desire of Steven(red), drugs and money mostly, and wouldn't respond to Steven's advances until they got vulgar(something anyone can offer); showing excessive enjoyment in drugs and sex as priority.  Thus the  failure of Steven(doesnt even know how to fill out the form, although he seemed genuinely confused, as if he really tried), leads Becky easily towards the number one need of finding drugs and money somewhere else, dont really think Steven alone without money or drugs would stop her from going elsewhere.  Steven as something like a different Bobby, one without the guidance of the major, cannot see that something is wrong with whats going on, gets caught where he shouldn't be, etc..  Also good to remember that they are married....maybe a clash with Steven and Red/Jean Michel down the line?....