"...Cocaine is the one drug that has been known to kill cardiac muscle cells on contact. Cocaine causes development of thromboemboli, tachicardia, and severe vasoconstriction!"
Cocaine is one of many drugs that can have negative effects on the heart, but as far as direct cardiotoxicity (damaging the myocardium), alchohol is much worse. Cocaine can cause heart attacks in otherwise young healthy individuals by causing vasospasm of the coronary arteries. This is the most common manifestation of cocaine abuse on the heart, but it is known that high levels of circulating catecholamines can also lead to cardiomyopathy. So, anything that causes elevated peripheral catecholamine levels can theoretically cause problems in the long term. Cocaine, amphetamines, and ephedrine do this. Intravenous cocaine users often have premature dilated cardiomyopathies, in which the heart becomes blown up like a balloon and can't pump properly, leading to heart failure.
As far as steroids, I've beaten this issue to death here, and there are always going to be the "pro-steroid" people who refuse to accept that the use of steroids can potentially damage the heart. It's one thing to continue to do something when you know it's bad for you, like smoking, but by propagating bullshit like "there haven't been any studies to PROVE that steroids affect your heart" you could be helping people to hurt themselves. To be honest, I haven't found any studies because I haven't taken the time to look. I don't need to have studies to form an opinion that is based on inference, existing data, and common sense.
Steroids raise the blood pressure primarily due to the stimulatory effect on red blood cell production. The more cells in a given space, the higher the pressure. This is a reversible effect, but high blood pressure increases your risk of stroke and heart attack by 30-40%. Draw your own conclusions.
Steroids also transiently affect blood lipids, usually in a negative way (unless you were hypogonadal and you supplemented w/just enough T to bring your levels to normal). We know that dyslipidemias are a risk factor for premature heart disease, so if steroids do in fact have a negative impact on blood lipids, you're only bringing yourself that much closer to a heart attack.
What is not proven, is that steroids have any effect on heart muscle size. However, it IS proven that athletes commonly have a physiological concentric hypertrophy of the left ventricle in order to compensate for the increased demand placed on the body during exercise. It's called "athlete's heart." Go figure. Again, if steroids do have hypertrophic effects on myocardium (which does not seem like a stretch since they affect skeletal muscle), you're only bringing yourself that much closer. I've argued that GH can potentially cause pathological enlargement of the heart, and people violently disagreed - "there are no studies, blah blah blah." GH-secreting tumors in adults cause a multitude of effects, one of them being visceral organ enlargement. The heart is a visceral organ. Do the fucking math.
As far as Arnold, there was controversy surrounding that. Maybe he really did have a congenitally aortic bicuspid valve (which is what one news source said), or maybe he had severe mitral valve prolapse. Doesn't mean that steroids or GH didn't contribute to his problem, and it doesn't rule out other heart problems secondary to drug use. We don't know, so there's no point in speculating.
What I do know, is that an acquantence of mine, a TON of pro wrestlers, and probably a few bodybuilders, have died from heart attacks at a young age, and they all used steroids. What was said about the typical lifestyle of the average person who uses steroids is a very good point, but steroids don't help the problem. If you're the kind of person who will continue to abuse steroids because there aren't any specific studies showing that steroids are a direct cause of heart attacks, you're not very smart. That sounds like the mentality of my grandparents' generation, who claimed that they didn't know smoking was bad for them. Ok, I personally don't see how you could fail to see inhaling something on fire as being bad, but whatever. Either way, it's not a smart gamble.