What's good big pimpin', I just read through this. My feedback is as follows:

First let's take a look at your opening, and let's see how we could improve.

This morning when I woke up, I didn’t think things could get worse.

Instead of using 'thought' verbs like think, believed, felt, I always try to instead expand upon what exactly it is that is causing the character to think or believe or feel something. So rather than 'The teacher believed Roger was cheating.', it would be 'There was often movement in the third row, second seat, where Roger Rimrock sat. But whenever the teacher would snap her head up, Roger was always straight as a rod. Something was up.'

However, this is a short story, and you obviously weren't aiming for a ton of wordcount. In a case like this, I would honestly drop the second half of the first sentence entirely. It would read like this:

This morning when I woke up, at the foot of my bed, was a guy I'd known all my life.

And then continue on just the same from there. We don't need our protagonist telling us he didn't think things could be worse, because finding Gus dead and soiled at his bed says it all for him! Stronger, and more immediate. In the same paragraph there's one of those 'thought' verbs again right here:

I knew he was dead because my favorite pen, the one my ex-girlfriend had given me the birthday before she found me and her sister in a parked car in front of her work, was sticking out of his right temple and there was shit in his pants. Cold shit.

Again, just drop the icky and unnecessary thought verb.

My favorite pen, the one my ex-girlfriend had given me the birthday before she found me and her sister in a parked car in front of her work, was sticking out of his right temple. And there was shit in his pants. Cold shit.

That gives the reader everything they need to know without being spoon-fed.

She seemed surprised, but she agreed.

This is another one I'd 'un-pack'. Again, I get it, short story and all, but it's good practice if nothing else. Give us a specific detail that'll clue us in on her surprise without outright telling us.

When I came to, a doctor was checking my chart.

I actually like the way this paragraph reads better without the phrase 'When I came to,' Again, there's more immediacy without it and the ensuing details will clue us in on our narrator's previous unconscious state. A few other notes on this paragraph:

A doctor was checking my chart. I had a headache like I’d been slapped by a whale. I had no idea what happened to me, but hell or high water, I was going to get myself a pint of Harpoon. There’s a whole lot of strangeness in the world and I’ll be damned if a cold pint can’t go toward fixing them.

A lesson on similes and clichés. Anywhere you can use a simile, a metaphor is usually stronger. This is a lesson I learned from those old Chuck Palahniuk essays on writing, in fact. So, instead of 'I had a headache like I'd been slapped by a whale.', it would be 'My head had been walloped by a whale.' Stronger. Too many little comparisons distract your reader and they're usually too weak to make much of an impact. In fact, just avoid using the word 'like' to compare two things in your writing in general.

As for 'hell or high water', this is a less important note than simile usage, but good to note nonetheless. Idiomatic phrases in small doses are usually pretty safe, so you're fine here. But, for practice, instead of using these packaged cliché phrases everyone knows, phrases like 'hell or high water' and 'a blessing in disguise', try re-inventing the world in a way unique to your character. Give them their own 'burnt tongue' and made-up idioms to use... because real people do that! This is an easy way to characterize people and make your story feel fresh and unique. Often times I'll go through spotting every idiom I slipped in on autopilot and replace them with phrases and verbal tics unique to the character. The result is almost always stronger writing.

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Something that worked for me in this story were the interactions you had take place between the characters. They were snappy and moved things along without being too wordy. As thenewgreen mentioned, this was very noir and that's always a fun, sexy setting to play in.

Keep writing.